H-1B is portable in theory, and many H-1B's to switch. However it is tedious and risky for many reasons. For example:
- There are many restrictions. H-1B is still coupled to a particular occupation (type of job). Any new job has to remain >50% similar. People on H-1B can also only go to companies that will sponsor their H-1B going forward (so self-employment and most smaller companies are out), and one would not want to go to a company that would not also sponsor the greencard. Transfers in the same company are possible, but even just changes to the job description (e.g. becoming a manager, or switching from devs to ops) or worksite usually require a new PWD (Prevailing Wage Determination) and LCA (Labor Condition Application).
- The processing of H-1B transfers has recently taken around 6 months for some. Theoretically you can switch before the government approves the transfer; practically if the transfer is rejected for whatever arbitrary reason or government error you want to be able to fallback to the existing H-1B. And so it is advisable to wait for the transfer to go through. How many potential new companies are fine with that? (What is the damage to previous companies from people quietly just coasting and waiting for the H-1B transfer to go through due to such delays?)
- Any change also increases the risk of rejections during the next renewal (and people from greencard backlogged countries have to renew every year, for decades at the current rate). How big the risk is depends a lot on federal politics (for example, the prior deference rule was withdrawn during the previous administration, and reinstated during this administration) and can change fast.
- Switching jobs while on H-1B during certain stages of the immigration/greencard process (PWD, PERM, I-140 plus 6 months for AC21 portability) resets that process, and restarting it incurs potentially years-long delays and large expenses (for the company, which is worse). PERM itself may have become harder recently due to changes in the legal landscape and one would not want to give up a successful PERM.
- The risk of layoffs is perceived as greater for new hires than for established engineers. A person on H-1B who is laid off has 60 days to find a new job, or the H-1B ends. So during an economic downturn giving up a solid position is a risk.