It's because people are lazy. It needs extra work to generate something non generic. Also a lot of people using AI have no sense of beauty, as without AI, they are not very creative.

Using stable diffusion on a1111 myself, with controlnet, regional prompter, different checkpoints, a ton of Lora and inpainting, one can create much much better stuff. It's not harder that way, just takes longer than copy pasting prompts and hitting the generate button.

I know this is true, because I see this daily by now. The amount of generic images uploaded to for example Civitai is proof of it.