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Extremely inefficient markets. Traditionally, VCs have been able to collude together to prevent startups from increasing in valuation "too fast". Unlike more efficient and less manipulated markets, which tend move in fast discontinuous jumps, VCs have been able to slow the top-performing companies to a slower exponential curve.

Because they've typically been able to slow this curve, waiting until the next round usually hasn't caused them to lose nearly as much upside loss as it would, were the market more efficient.

With IPOs being pushed back, and more sophisticated IB investors moving into much earlier rounds now though, this is finally starting to change.

Keep this in mind whenever you hear VCs shouting "bubble", just because some top startups aren't following the smooth exponential curve in valuations, that any competent trader or economist could tell you should never exist in an efficient market.