The declared intended purpose of H-1B is not for the benefit of the worker but for the benefit of US economy - the visa is intended so that the US employer can fill that particular job position that requires special skills.

If that position ceases to exist or gets filled by a local worker, then the visa loses its purpose, the employee apparently isn't needed anymore by US and can be discarded.

I imagine it's to deter a certain kind of exploit. Hiring Quantum Rocket Surgeons and a week later they're Surface Hygiene Associates being paid much much less. Or vice-versa.

It's to prevent a bait-and-switch on behalf of the employee and situations where a person is "employed" by a company only so they can immigrate and then do something else, on behalf of the employer.

We can easily let people switch job titles and employers while addressing potential loopholes though.

I think the idea is that the H1B isnt a general 'work visa', it is given because there is a position at a company for a worker with a specific skillset. So it makes sense in that framework. I'm not arguing that this is the right way, just what the reason probably is.

You have to see how laws are made in the US. They are written by lobbyists/corporate lawyers (the lawmakers have stopped pretending otherwise these days). For the company, it totally makes sense. They spent something upfront (H-1B fees and uncertainties are non-trivial), so they want a guarantee that this person cannot change jobs easily. That the lock-in is for 6-years is just pure power-play.