To start with, you have to first correctly guess all of the groups; you can't get all matches correct if you don't know which matches those are. So that's not super likely. There are 32! / (4!^(8)8!) ways to draw the groups (assuming we don't care whether a given 4 teams are in group A or B or C etc.) So already that's a lot. 1 chance in 5.6 x 10^19 to get that correct.

And now you have to start guessing the outcomes correctly. There are 48 group stage matches so that could be (1/3)^(48) possible outcomes. So that's another 10^23 outcomes.

The knockout stage participants are not critically dependent on the specific results of each group-stage match, but it's highly correlated. So you have 16 teams in the knockout and those are win or lose, so you have (1/2)^15 chance of getting all those right. That's much much easier than the other two stages.

If you wanted to have this remain in the realm of possibility, just do a knockout stage prediction. That's already extremely unlikely, whereas the others are completely impossible from a practical standpont.