One reason why investors may be going gaga over the recent mini-flurry of IPOs (Uber, Beyond Meat, Slack, etc) is the simple fact there are so few of them.

Point No. 1: There are a handful of theories as to why there are fewer IPOs these days, but I’d like to highlight a key reason that I think is greatly underrecognized. Hint: It’s all about ecosystems.

Point No 2: Fewer IPOs is not a good thing, again, for reasons not so obvious. Hint: It’s all about inequality.

Before I explain, here are the numbers:

Up to 230 companies will likely go public in 2019 if the market doesn’t tank, Renaissance Capital predicted earlier this year. And while the number of IPOs has been climbing some over the past several years, it’s actually still on the low side historically. From 1980 to 2000, an average of 310 companies went public in the U.S., according to Jay Ritter, a professor at the University of Florida. (In boom years there were hundreds more.) After 2000, the yearly average dropped to 110.

Venture capital and M&A madness

Why is that?

First off, the IPO pipeline went bone-dry after the dot.com crash of 2000, but truth be told it never really recovered after that.

There are a number of generally accepted theories why fewer companies are going public. One big reason: They don’t have to. Silicon Valley is awash in money. The venture capital business used to be a fairly small marketplace, but no more. For a number of years now, institutional investors have been flooding in with cash. “Private investors poured $130.9 billion into technology and biotech companies last year, far outpacing the $50.3 billion raised via IPOs and follow-on offerings,” writes Jane Leung, chief investment officer at Scenic Advisement, an investment bank for private tech companies in San Francisco. Leung notes this outpacing has been going on for a decade.

No wonder the “unicorn”— a private tech startup worth more than a billion dollars — has become emblematic of this Silicon Valley boom. Telling too, that the metaphor was coined by a VC, Aileen Lee, in 2013.

But there’s another, underappreciated, reason why there are fewer IPOs. And that is simply because so many hot startups are being gobbled up by big tech companies. This isn’t a completely new phenomenon. Back in the 1990s, companies like Cisco (CSCO) were positively M&A mad. But there’s something very different afoot now, and it has to do with a major strategic impetus in Silicon Valley these days, that being the creation of so-called ecosystems.

At its core, an ecosystem is a business model where a tech company looks to serve as many customers in as many ways possible — and even beyond that, one part of the business helps grow the other. Of course in the digital world the potential for a company to jump into one business after another is almost limitless. And so Google (GOOG, GOOGL), Amazon (AMZN), Apple (AAPL), and Facebook (FB) — the Murderers’ Row of ecosystem companies — expand from one market to the next; from search to music to ecommerce to payments to delivery to driverless cars to drones.