In practice you're right though. Most companies have limited experience with this process and just don't want to get into it.
- The green card process doesn't completely transfer over.
- Filing a petition to transfer an H1B puts the employee at risk of getting a denial. Every petition sent to USCIS is risking an audit, denial, and at the very least, many headaches.
People are making offers to candidates with undefined start dates many months in the future. The candidate has to commit to switching jobs when the transfer is submitted. If current job gives them a promotion in the interim and they decide to stay, they can’t just cancel the transfer. Their current employer (who didn’t know a transfer was happening) has to file a reverse transfer with the same delay that puts them in some kind of limbo.
It’s a complete clusterfuck and is making it really hard to hire for some positions.
I went through 2 H1B "transfers" (so in total I have been employed by 3 different employers as a H1B) 7-ish years ago so I'm very familiar with this process. At least, the way it was working at that time.
This also addresses the concerns about salary differences between different industries and regions.