At the end of the day, they have money. Assume they are selfish and want to not lose money, so they probably have some rationale on which they give out money in hopes for a return on investment. That being said, I've seen way too many tech bros with bad blockchain ideas get way too much money which usually goes straight to their heads before they lose it all, and I assume the V.C.s have a really good strike price and know exactly when to liquidate.
For others, it was my general understanding VCs lose money on most investments, hoping that a few end up going really big and canceling out the losses on the rest, but I don't really know anything about VC stuff. I'm not sadomasochistic enough to try to start a company, I just try to work on my skills and people are willing to pay me to invest in myself because I tend to discover interesting things while I tinker about and also keep things from failing often in a raging dumpster fire but I don't know if thats a good way to market my skills.