Many things show a Trainer's Pokémon are better off than Wild Pokémon:
Wild Pokémon can't learn moves other than Level Up moves with the exception being some Egg Moves.
Some Pokémon evolve by being traded so they need a Trainer to trade them.
Some evolve directly from Friendship.
Since the start of the series, it's been shown Pokémon can disobey their Trainers if they want.
Out of the four "gimmick" mechanics which are Mega Evolution, Z-Moves, Dynamax/Gigantamax and Terastal Pokémon without Trainers can only do Dynamax/Gigantamax and Terastal and even then they work differently than they do for a Trainer's Pokémon.
While it's kept ambiguous what it's like for a Pokémon in a Pokéball the description for the basic Pokéball since gen 6 mentions it's "comfortable".
I guess this all also depends on how smart the average Pokéomn is.
There is also this:
https://bulbapedia.bulbagarden.net/wiki/Dr._Footstep/Quotes 