1) It's the amount you can borrow from your family most of the time. Or just make it working in IT and saving like mad.
2) It's barely useful. $150K is not enough to hire even one great developer for a year.
2) -We are investing in team when at least one founder is a developer if not all... So $150K is not to hire people. -All developers are not in the Silicon Valley. You can find developers at much lower price in many countries and we are investing everywhere.
Time = Money
Twitter & FB are easily reproducible, and they provided awesome returns for their investors.
Some people have financial obligation or living situation where they could save like mad and it would still take them years to accumulate 150k.
Barely useful? This is that SV bullshit mentality. There are thousands of great entrepreneurs all over the world who could turn 150k into a profitable little company. Might not make millions in the first few years (or ever) but if they love it and people love their product/service, that is very useful. Surviving off very little is an admirable characteristic, you don't need millions to build something meaningful and if that's the way you think, I would never work with or invest in you.
Alas, VCs are not after profitable little companies. They just plainly would not allow you to become a little company, profitable or not.
It's also easier to raise a big round after that, invest a lot and grow.