I don't agree with the conclusion that it's because it's peer-to-peer. that's not why -- it's because of lazy developer methods and a lower prioritization of security effort.
the biggest genuine effort that Rockstar puts into anti-cheat effort is an occassional memory-structure shuffle to kick sand into cheat-engine users eyes, and the occassional honey-pot that bans a few hundred people -- and these efforts come after begging Rockstar for years to do something, and the most it accomplishes is selling additional copies of the already dirt-cheap game.
These ban cycles and 'enforced ignorance' to the problem nets them more profit than it would otherwise; disenchanted players play less, reducing infrastructure costs -- and banned players buy another 2 dollar copy of the game -- but it kills user experience outside of single player entirely.
They don't care. I get it, but it sucks -- and it's not some GTA6 thing, they never cared.