>>540840861. I like my Pokemon underleveled for the gym challenges so they aren't me clean sweeping with one move. Having to rely on strategies makes fights more fun.
2. I don't want my overleveled Pokemon to stay overleveled. I want to park them for a little bit, while still keeping them in my party in case I need them, without them getting EXP
3. EVs are shared by the EXP All, meaning that you can only raise one Pokemon at a time unless you want your special attacker to get defense/attack EVs and your bulky physical attacker to get SpA EVs.
There's a reason that modern, high-quality RPGs don't have EXP All. Octopath Travelers 2 takes it even farther by making it so you get no EXP or job points if you're dead when an enemy dies. Not only does your benched party receive no EXP/job points, the dead don't either even if you revive them by the end of the fight.