Zero days are plentiful. But there are only a handful that you could buy today which could potentially give you access to a CEO's phone. The only other option is to build your own team to find a zero day for you, which is not cheap or quick.

A better saying is that zero days are... fragile?

You use zero, someone notice, it gets fixed.

And yes, they are rare compared to the vast # of known vulns.

Ideally if given the choice, you'd utilize a known vuln against outdated software rather than risk burning your zero day.