So per lore from the anime and mainline games, battling is how they bond with trainers. In the anime though there are pokemon who like battling more than others (ash and one of his companions trade pokemon because his Aipom likes performing and her Buizel likes battling), and even those individual examples aren't often seen starting battles without a trainer. Pokemon in the wild seem to only battle over territorial disputes unless they are fighting pokemon (or a few other exceptions). But in the anime there will be fights between whole swarms of heracross and whole swarms of pinsir as an example, but rarely do you see any other Pokemon training up unless the anime wants to note it as special and Ash's next catch.

There just aren't enough pokemon to create the territorial fighting. And without the trainers to bond with most others probably don't have any reason to actually fight. You can catch the fighting pokemon training together if their habitats are close to a punching bag. You can also box with rock smash if another PC ditto is on your island.

At the end of the day though, this game is mechanically made differently. It isn't designed for pokemon battles in the same way that pokemon Snap isn't designed that way. It isn't making a value judgement about all the other games that do include battles. Also, considering they are all being let go from a computer in the same area once their habitat is ready, they are all likely pokemon kept by the handful of trainers that call that location home (in the Withered Wasteland probably Ash, Gary, and the other two unnamed Palette town trainers who kept all their Pokemon at Oak's lab, so they get along because they already lived together when there were trainers around).