I think it has to do with desired quality. Hiring high quality engineers can be a difficult task even in the massive talent pool present in SV, and it’s only going to be more difficult in cities with smaller pools. That’s not to say that quality doesn’t exist in these places, but it’s certainly not as readily available.
These other major tech-centers have less well-paying competition than SV as well. As someone who has done hiring in several companies in Berlin & Vienna the difficulty mostly lies in the fact companies don't want to pay much.
If you are willing to pay 10-15% above market rates (which is still 1/2 of SFBA market rates) there is no shortage of senior engineers in Berlin.
