>>52678496You don't always, get a worse mon, though I will agree it's hard to fuse for raw stats most of the time, you fuse in ways that don't give you terrible base stats but give you a good a good typing + hopefully a useful ability. If you understand how fusion works, you'll look out for the mons with really good stats for it, and that's not even getting into the fact you get to pull from 2 movesets.
TLDR mons with Good HP, Special Attack and Special Defense make good heads, and Mons with Good Attack, Defense and Speed make good bodies, often times as long as it has good non-offensive stats for the side you're using it on it will do great.