Is it that SV devs are that much better (perhaps the best talent agglomerates there)? VC's like being in the same time zone as their companies? Even if I were developing for just the US market it seems like a strong arbitrage opportunity.
But for startups in particular (which I assume is what you really mean, since you mention VC), I think there's more reluctance to employ that kind of contract help, especially in the very early stages. The thinking seems to be that you want the people building the early version of the product to be all FTE's, probably with some equity, sharing the same vision of what the company is trying to do, physically co-located, etc. Now I'm not saying that it's actually the case that those factors are important for startups... just that a lot of people seem to think that they are.
Me personally? When we get to the point of needing additional help, and assuming money is available for paying help, I'll almost certainly be looking to some of the Ukrainian firms I've worked with in the past. But that's just me. shrug