You're really comparing two different things here.

Prometheus is great at alerting. So, where do those alerts go? How do they reach the right team, the right person who is on-call? How do they know that Person A is on-call between 8am and 5pm, but Person B's shift starts after that? Oh, wait, Person B is on vacation and asked Person C to cover for them.

Person A, B, and C's manager wants to monitor how the MTTR on all of those alerts is. Where does Prometheus provide that data?

These systems don't do the same things. OpsGenie isn't just an "SMS Gateway".