[misnome]

This is a good question! In reality, for this process they do: see https://en.wikipedia.org/wiki/Pair_production. But ignoring this and JBorrows (correct) comments on other properties to be conserved, we can ask:

IF more matter is made (in some process) than anti-matter, then why? To explain this, we would need some sort of mechanism where matter was treated differently to antimatter in the laws of physics... and then you are back to the original question of looking for differences.

Non-gravitationally, the general case of antimatter behaving differently to matter is covered by the concept of https://en.wikipedia.org/wiki/CP_violation - we do know of cases where this is the case, but not strongly enough to explain what we observe in the universe - the article looks a little heavy but does have a section on the matter-antimatter imbalance that is a little easier to read.