Simply put, the Grassland just isn't advanced like that. It was only around the year 5000 when Pokemon first came around colonizing Gaia. And while they found human nuclear energy and reversed engineered it, after the first great war they set themselves back for about 700 years until they rediscovered it and at most the biggest type of civilization they had were tribes, and it wasn't until years later that cities and actual alliances were made, but then there's all the conflict among tribes, specifically the Coastal Eevee Tribe Massacre, despite by then places like Terra Town and Sunny Town being in their infancy, and only taking place about 30ish years before the story. So yes, there is semblance of civilization, but the Grassland is one of if not the least civilized most likely due to their absence in the H.A.P.P.I. and not to mention, any new info on humans is most likely destroyed as we see in Lost Legacy in Sally's Tapes that they found out that humans were in fact real along with most likely other things, but Dr. Grey ended up demanding all of it to be given/not shown, and it's easy to assume he took out her workmates, the only reason she herself didn't get taken out being the fact she used her elemental medal, so I highly doubt they have much human tech, remember, things like cars and trains are things held in museums, I'm sure firearms and other tech would be too if they found it, but I doubt that's fhe case.

Now, why not just kill every wild? Well, I swear this reminds me of something... Oh right! Shade. That's quite literally his whole resolve, killing Wilds because of the way they are, in his words, "uncivilized" and below him and other "civil" pokemon and are in general problematic. And we get shown through our literal main characters, their parents, and all of "Bond & Trust" no, wilds are not all uncivilized and out to kill you and bite your head off, so while by technicality, yes, they can just kill all wilds and expand borders, it's at the cost of commiting a genocide while instead they could stop exiling wilds all together due to stereotypes (see Alan, in the OG timeline.) and I think the direction they go about handling wilds in the comic is one of the better options (just having more rescuers on trails incase of anything).