I don't think it's that uncommon. I have personally worked at two companies that didn't do the background check until after employment (which I would never do again), and I've heard this anecdote a few times from others.
hmm I know for some position there has been a high first day no show rate, I wonder if that is what drives that position? Background checks while not super expensive are not cheap either and if you have a high number of people noping out before the first day I could see where they would want to mitigate those costs.
With the changes in drug laws, and this I have seen some companies starting to forgo pre-employment drug screens, which IMO is a good thing.
I am not sure what value most of these Pre-Employment checks provide anyway really.
These were US-based startups, to be clear. I would be surprised if very many larger companies did it this way since they generally have a formal process for this.