Then you end up with a whole map where you can only go one or two places to be challenged. Huge areas that are completely pointless and offer no reward, instead just having to go and do the same area over and over again. How boring.
The feeling of getting stronger comes from the gear you find and working out ways that you can get the powers they have to synergise with each other.
I’d much rather have it this way. As an example, I played starfield a while ago and even though it has dozens and dozens of star systems you are stuck with just 3 or 4 that offer even the slightest challenge at the later levels.
Edit: That’s not to say that levelled areas don’t work in other games of course. I love to play Valheim and that has levelled biomes but it’s a completely different game. It’s a journey with a start and end (end still not done though). It’s meant to be beaten, finished. Diablo isn’t like that, it’s a perpetual game. Elden ring is another example but again is a game that’s meant to be beaten.
I can see quite a few trees if I zoom in on the image, they’re just young saplings at the moment as it’s all new and just been landscaped.