Is it really unspoken? I mean VCs aren't going to advertise it, but I thought this was common knowledge.

Fun anecdote I heard from a very seasoned VC. He mentioned a trick VC funds use is to raise capital, then spend that fund before any of the companies can make a return (this varies from fund to fund, but imagine anywhere from 2-10 years, usually centering around 5-7). They then go back to their LPs saying "look how good our portfolio is doing, we just need more money to help them out!" without any truly meaningful metrics such as profitability, and essentially keep the house of cards afloat like this by cyclically raising capital before funds can show an ROI.

Also most VCs lose money, but nobody (by which I mean LPs) ever seem to care about that because they're excited about the potential without understanding the risk of the market.