It may not be hard, but there are no guarantees and a clock is ticking.

Say you lose your job. Boom, the clock starts ticking and now you're in a rush to find an acceptable job that meets the criteria of H1B.

Say your manager mistreats you; denies you promotion, raises, etc. and you want to leave. You can't just quit and start looking. You have to line up something that will come through in 60 days and only then you can be free to quit.

None of these situations apply to citizen or PRs. It tilts the balance definitely to the employer's advantage.