They don't need to tap public markets to raise money. And investors are likely OK holding off on liquidity if the valuation continues to rise.
Going public typically requires a significant amount of effort across the company, which can reduce focus on other initiatives key to growing the business.
It's not necessarily the same price you'd get on the public markets, but it can help give employees some liquidity.
If their fundamentals growth slowed to rates resembling the regular market, then sure, but as is, all a company with that growth rate might need is let tenured employees add, say 5% of their equity to the round so they can get a bit of liquidity to keep them quiet.
I'd rather take 2x whatever today than 0x nothing when the company folds tomorrow/in a month/in 5 years never having gone public.