It would be relatively easy to build the logic for on-call rotations, and to build the API endpoints for consuming alerts. But add to that the work of also building mobile apps, working with SMS and email infra, and this becomes a non-trivial effort. For a company like Uber it might make sense to dedicate a tiny team to build an in-house solution. For small startup, it still is not worth the hassle to maintain a service like this in house.

Bonus question - how do you alert your team that your home-build PageDuty went down?

I would guess they kept a small amount of SREs with Pagerduty access (or other third-party service) to monitor this system, or built another simpler alert system to monitor it.