The problem is how much computation is required to handle every user request.

When the Internet was starting out, most websites weren't much more than text, maybe some low-resolution pictures. Even in the '90s, serving that content to users was computationally cheap. A company's web server could just be a desktop in the basement.

AI models are expensive to train and expensive to operate. Just maintaining the environmental needs for the massive data centers is a significant cost. Charging users for access is not nearly enough to cover the expenses, by orders of magnitude, and they're already in massive debt.