Sqreen (YC W18) is one counterexample, but they came into YC at a later stage of maturity than your average YC company.
As to why there aren't many others, that's a great question. It's a hard space to get started in, since security is mission critical for essentially everyone, so that likely doesn't help matters. There's a lot more that needs to go into a viable MVP in cybersecurity than in other areas as well, since there's much less room for error and jank.
Those reasons likely contribute, but none of them are hard blockers.
As to why there aren't many others, that's a great question. It's a hard space to get started in, since security is mission critical for essentially everyone, so that likely doesn't help matters. There's a lot more that needs to go into a viable MVP in cybersecurity than in other areas as well, since there's much less room for error and jank.
Those reasons likely contribute, but none of them are hard blockers.