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!