I am almost certain their investors want them to aim at least that high.
Are you sure you're not confusing billion and trillion?
ADDED. I'm not trying to appear bad-ass or hardcore (and in fact, I've never been a founder because I judge that I cannot afford that level of risk of being left with a severely suboptimal monetary reward for my efforts). I am honestly trying to understand VC-funded startups.
50 billion 4 --- 10 billion : 100 --- 1 Billion : 10,000
But to answer your question and being a startup founder myself - doing a startup is just an amazing experience. It's a bit like trying heroin (not that I've done that, but from what I hear). The highs are extremely high and the lows are extremely low. Going back to a "normal" job after doing a startup is like taking methadone.
Sometimes I wish I could think rationally around this (like you) and just take a normal job again. That would most likely be better for me in every measurable way (salary, benefits etc) but I'm just having too much fun doing startups.
However, I do not find life easy (nor am I young).
They're definitely not only aiming for $1bn+ exits although obviously they're a bonus (in the last decade there have probably been under 50 billion dollar exits in the tech space).