VCs consider the entire context around their would-be unicorns. They don't invest solely on the founder's determination. They also consider the plausibility of the idea, the market conditions around the proposed business, the estimated amount of resources needed to compete, etc. It is NOT solely based on one's determination to be "a unicorn", though it's nice to have a determined founder.
People who believe that will is everything are generally pretty naive. Successful people know that if external conditions were even slightly different, their path wouldn't have turned out the way it did, and no one can guarantee it would've been as prosperous.
For example, I make an OK living as a software developer. My interest in computers started when my dad brought home a Packard-Bell in the early 90s. He bought it through a discount program that his workplace offered to increase computer ownership amongst their employees. If he had worked for a different company, or if his company had not made it affordable for him to buy a computer when they were still relatively uncommon in homes, it's very possible that I would be out there bagging groceries somewhere.
If MySpace had been a little wiser, there would've been no room for Facebook. Did the people at MySpace feel the sheer force of Zuckerberg's unicorn-desire and decide to just back off? I don't think so. He was in the right place (at least half-random) at the right time (80%-ish random) and had the right skillset (probably 10% random). There are many thousands of kids like him that didn't have "the stars align" when they had their shot. It doesn't mean Z. wanted it harder and it doesn't mean that these no-names didn't do a good job. The world is not a meritocracy.
95% of YC-backed startups fail. A few go on to have decent success, and maybe 2 or 3 YC ventures actually went on to billion-dollar valuations (Dropbox and Airbnb). And that's considered a really good success rate.
Since we're talking here about what you call 'unicorns' (I hate that word), then we're talking about the odds of building a billion-dollar company. YC is made up of hundreds of incredibly talented entrepreneurs, and even then, it's pretty evident that building a billion dollar company is as much about luck and circumstance as it is about talent and determination.
EDIT: Correction - I was very wrong about YC stats, as it turns out [1]. My mistake. I stand by the original point, though. 8 billion-dollar companies out of ~940 YC companies demonstrate clearly that building a billion dollar company isn't easy, and takes a whole lot of luck.
95% could be accurate. I'd consider every "acqi-hire" an overt failure. Your business failed, but you personally got to fail upwards. acqui-hires are generally VC-engineere soft landings (everybody has friends they can ask for $50 million in a pinch, right?).
Just because money changes hands in the end doesn't make it a success. You fail upwards until you become a creature of pure thought leadership (i.e. a VC whose only job is to 'have an opinion' on stuff and can never be proven wrong because, hey, it's all an opinion or luck or the market or some unforeseeable outside force you failed to consider when failures continually happen).
They're fairly explicit about it.
And their track record is pretty positive about the concept.