Kind of what's going on here is that only very successful companies face a large enough impact from the tax that exercising the option of exit is at all attractive. Say your SF startup was being acquired for $50mm, $10mm of which is employee equity hit by the tax (investors and founders don't pay it). You end up having to pay $150k between you, which only now is starting to be a big deal -- even one order of magnitude down, it's not worth the hassle of moving your office to a low tax jurisdiction just to dodge 15k of taxes imposed over 2-5 years -- and of course you have bigger issues to worry about so the tax doesn't matter that much. Spread over 100 or so employees and it looks like a fairly small salary increase to be paid for cost of living reasons.

I faintly recall a discussion of when you should accept dilution (which a tax is) if it raises your chances of success. I tend to assume that startups locate in SF rather than the non-SF bay area for reasons that they feel give them more than a 1.5%*employee equity higher chance of success (shorter commutes for your people, etc.).

More generally, it's only going to be the very large companies that want to exercise their option of exit for this reason: moving sucks and nobody wants to do it. By the time it's millions of dollars in taxes (i.e. valuation > $1bn or so) you'd think about it, but I'd guess not before. So it's not the greatest idea from a tax equity perspective: a tax only the extremely wealthy find it worth their while to dodge is still regressive. Lame capitulations to keep the few companies with this high-class problem in the city are probably correct from a utility-maximizing perspective, if kind of morally distasteful.