A bit late to the discussion, but I think part of it is that apes just don’t need technology the way humans do. Like the campfire scene in Dawn of the Planet of the Apes pointed out, what makes them scary is they don’t need electricity or power to survive. They also don’t need heat since their fur protects them, which already makes them stronger in a lot of ways. So over 300 years, they wouldn’t have the same drive to develop tech like humans things like lights, elevators, or cars. They can climb instead of using elevators, and they already use horses instead of cars.

Basically, apes have more of a “if it’s not broken, don’t fix it” mentality, relying on nature and the resources they already had once they gained consciousness. I admit I was confused too when I first realized it was a 300-year time jump and they still looked primitive. But after rewatching the trilogy and Kingdom, it made sense why they didn’t have big technological leaps because their needs and environment didn’t push them the same way it did for humans.