Because building on top of LLMs is
really tricky. You need to understand things like writing evals, configuring agentic loops, creating and iterating on system prompts, designing tools that work well with LLMs.
It's a speciality, just like being a payments engineer who integrates with systems like Stripe is a speciality.
Being familiar with agent-assisted development helps a little bit because at least you understand prompts, but there's a whole lot more to building software on top of LLMs than that.
Any engineer can get familiar with these things of course, just like any engineer can figure out what it takes to work on payment systems.