actually, if you really look at it, there aren't many Pokemon trainers.

most people you fight have jobs or have other interest than being a Pokemon trainer, but they do however see you as a trainer and think "well I got a bit of free time in my schedule, this young one does need some training, I will fight them"

they seem to be only doing it for your sake and not for theres.

let's be fair here, to have all the NPC's live a full scheduled life including routes and homes to go to, the game would be on a console. they are NPC's, they are designed to sit there till the end of time. but in the game story tells us that a lot of them do live other lives then standing there.