A few reasons, "AI" as used by non-experts often has correctness and security issues. Even when it doesn't, its outputs are often not reproducible/predictable because they're probabilistic systems.

AI systems are also prone to writing code which they can't effectively refactor themselves, implying that many of these code bases are fiscal time bombs where human experts are required to come fix them. If the service being replaced has transactional behaviour, does the AI produced solution? Does the person using it know what that means?

The other side is that AI as an industry still needs to recoup trillions in investment, and enterprise users are potential whales for that. Good prices in AI systems today are not guaranteed to last because even with hardware improvements these systems need to make money back that has been invested in them.