I think this will get more common with remote working. Start a new role, then dump it if one of the other searches turns up a better one.
I think FAANG has something to do with this too. They're often known to be slow in coming up with the offer, often seeming like they're not going to proceed. A friend started working at a startup, then got a top Google offer, dumped the startup for the better job that came in later.
Let's see if you say the same thing when you interview 10+ candidates and you accept one like this, rejecting all the others since you filled the position.
No different from the firm rescinding the offer once the candidate has turned down all his others. Plus it's asymmetric, the firm has other staff that can somewhat do the work, the employee has no income if the job is rescinded.
I think FAANG has something to do with this too. They're often known to be slow in coming up with the offer, often seeming like they're not going to proceed. A friend started working at a startup, then got a top Google offer, dumped the startup for the better job that came in later.