Why does math make no sense to me?

Definitely look into dyscalculia.
i don't remember how I learned those things, but the reason why people can perform the calculations fast may be because they just learned the results by heart since they were under 10. Maybe it's because you quantify everything all the time. Most of us just don't imagine 5 apples merging with another 7 and just remember that it's 12 straight up I guess.

I'm no expert, take my words with a grain of salt
This doesn't mean you are bad at math. You are bad at arithmetics, math is more about logic and ideas than just playing around with numbers. Sucking at multiplication shouldn't discourage you to learn some actual math in the future if you wish to try.

Practice helps, dont beat yourself too much over it. And ultimately arithmetic skills isnt that crucial if you can understand the concepts behind it.

For example, if a sweater costs 140usd and it has 30% discount, you can do the arithmetic with a calculator if you understand what the mathematical meaning of percentages is and how to work with them. Good luck!
Math is not easy, simple as that.  Everyone thinks about not just math, but everything in a different way, and everyone's brains are slightly different and more or less capable at a multitude of tasks than everyone else.  You aren't necessarily dumb just because you can't easily do addition, you don't have to add to not be dumb, that's not written anywhere.

As far as why, we know so little about the human brain and learning in general that it's almost impossible to answer.  I can say from personal experience though, I sucked at math in grade school.  The reason was that I felt like I was just expected to remember these random things, and there wasn't a way to do any of it without complete memorization of everything involved.  Multiplication tables were the worst.  I especially couldn't understand why the number 12 was so damn important, like why do I need to remember what 12 times whatever is, but screw 13 times anything. Why do I have to just spit out what 12 times 9 is, can't you just teach me how to *do* 12 times 9, and then I won't have to remember all this arbitrary junk?

Then, when we got to Algebra and started using variables, things started clicking for me for some reason.  I wish I knew what it was, but almost instantly I stopped just doing the rote memorization thing and I actually began *understanding* what I was writing down on the paper.  Around the same time, someone taught me what binary was, and even though that sounds so fundamentally different from math skills in general, what it did do was really show me what counting actually was, and why the numbers we write down mean what they are.  He said "imagine how you would count if you only had one finger, each time you got to two, you'd have to write down a new number to keep track of the zero and one you'd already counted, and for whatever reason that made all the sense in the world to me, and I suddenly understood, to use your example, why 5 + 7 had to have two digits in the answer, and why the first one had to be a 1.

Those kinds of discoveries just kind of unlocked everything for me from then on, and taught me how to learn things in general.  To this day I am really bad at memorization, but once I really understand something, it's there forever.  Unfortunately this probably doesn't help you at all, this is just my experience, and maybe this isn't even close to your particular issue.  I've just found that inevitably sometimes I just get stuck on something and I end up just missing this key piece that makes it all *make sense*, and as long as I can get that I'm golden but if I don't, I feel the exact same frustration I'm sure you feel now.  I hope you got something out of this, and wish you luck going forward!  Don't give up, the only way you'll never crack it is if you stop trying!
There's a chance you may have dyscalculia, but that doesn't mean it's impossible for you to learn math! If counting on your fingers tends to be a reliable strategy for you, there are ways to build off that strategy before jumping into the more abstract techniques. I'm a math teacher/tutor, and when I work with students who are in a similar situation, we tend to use manipulatives - tactile objects that you can pick up, move around, group together, etc. - to focus on the fundamentals of arithmetic. It tends to work well for individuals who still count on their fingers, because using these additional objects is kind of like having even more fingers to work with (some of the more common manipulatives are small blocks, such as Unifix cubes, or other sets that may come in ones, tens, hundreds, and thousands). You may end up becoming more comfortable with simple math equations by using physical objects as representations (e.g., seeing 5 things combined with 7 things, rather than just trying to conceptualize "5+7"), even before you try to work out solutions solely on paper or only in your head.
Math is not made for everyone. You can always keep doing it though, the brain is quite adept at adapting to information and maybe you will get better.
If you struggle with basic math operations, you're amongst 99% of world population. Everyonr struggle with multiplications. Some struggle to multiply single digit numbers whereas some struggle to multiply 4 digit numbers. The good news is however, we've invented calculators to do these operations for us.

Basically, you're struggling with computations. Not math. That's completely okay. I would suggest try to study some  geometry. You might like it.
Might try to get checked for dyslexia.
The way my arithmetic thinking sorta works is 5 + 7 = 5 + 5 + 2 = 10 + 2 = 12. Always trying to make it a nicer form so the numbers fit together and its easier to compute in my head.
I'm an engineer and in my studies I had to take math up through differential equations and linear algebra. I still count on my fingers because my brain struggles to perform simple arithmetic like addition subtraction and basic multiplication.

Some people are just wired different

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