That's just a farmiliarity thing. I've worked on project doing full web FE, mobile and BE.

It's hard to generalize but modern frontend is very good at isolating you from dealing with complex state machine states and you're dealing with single user/limited concurrency. It's usually easy to find all references/usecases for something.

Most modern backend is building consistent distributed state machines, you need to cover all the edge cases, deal with concurrency, different clients/contracts etc. I would say getting BE right (beyond simple CRUD) is going to be hard for LLM simply because the context is usually wider and hard to compress/isolate.