For wild Pokémons, it's probably for dominance. If they don't force themselves to evolve earlier than others, how would they show that they are fit to be a leader? Caught Pokémons are way different. Because they have a trainer to help them get stronger, they don't need to evolve as early to show that they are strong and can fight.
As for trainers with lower levelled Pokémon, maybe they were forced to by the trainer? Look at Ghetsis or Mars. Ghetsis has a Lv. 54 Hydreigon and Lv. 16 or so Purugly. Because these two Pokémons were owned by members of an evil organization, they had to evolve early on to show their true skills and impress their masters to show that they are not worthless?.... Or it could be for convenience. Can you imagine fighting Ghetsis with a Lv. 64 Hydreigon, making your team WAY underpowered? Plus, no one else had Hydreigon after the game, so GF just decided to lower Hydreigon's level for Pokédex's sake.