There are all sorts of reasons private tech companies with multi-billion-dollar valuations are putting off going public longer than they used to. The requirements of public reporting impose a big administrative burden on a company and eat a lot of its executives' time. Staying private means less scrutiny and fewer stakeholders to answer to. It's easier to pursue a long-term strategy when you don't have to worry about quarterly performance. 

But one big reason startups are staying private longer -- and why big tech IPOs like the ones Square and Match are doing today are so rare -- is that Wall Street just doesn't think the most highly-valued tech startups are worth as much as they think they are. 

Leaked financials from Lyft, obtained and published this week by Bloomberg, suggest that skepticism is well-founded. Despite being a clear runner-up to Uber in the ride-hailing business, Lyft is one of the biggest consumer-facing startups of the last few years. Everyone knows the pink mustache. 

Now Lyft is trying to raise a reported $500 million in venture funding at a new valuation of $4 billion. Yet the documents it's showing to investors to justify that rich price tag show the company losing vast amounts of money and not clearly moving toward a day when that won't be the case. In the first half of 2015, Lyft lost $127 million and only booked $46 million in net revenue. 

Moreover, those losses are accelerating: In the third quarter, Lyft was expecting to lose $124 million. The documents, which Lyft isn't commenting on or confirming, shows the company missing its previous projections by wide margins. 

The financial ugliness hiding under the hood of a unicorn like Lyft helps explain why a startup like Jack Dorsey's Square had to choose between essentially taking a markdown on its last private valuation, a move that required issuing extra shares to late-round investors, and not going public at all. 

The good news for startups, such as it is, is that this dilemma is already becoming a thing of the past. As Union Square Ventures partner Fred Wilson notes, the alternative to going public often involves taking money from mutual funds and other financial entities that have stricter reporting requirements than traditional venture capital firms. That means markdowns are now more often being forced even on companies that choose to stay private. While no startup welcomes taking a haircut on its valuation, at least it might make the prospect of an IPO afterward less painful.