I understand that. Course the other thing is that the tester was slacking off. Just being objective.
Doesn't happen for thee main reasons
1: there are usually a load of testers, so what one doesn't spot another will. Developers however tend to be given a specific area to work on. So if a tester slacks off one day or isn't top of his game someone else will generally pick it up, if a developer slacks off or isn't top of his then the project suffers as a result because there isn't someone to back him up
2: games testing is an entry level position which many use to try to break into actual development, slackers don't tend to be given many opportunities in any industry, let alone one that requires the sort of work ethic that the games industry demands
3: the vast majority of testers are in temporary positions only so that they can be downsized immediately after release. The other side effect of this being that they can be dismissed without reason or notice and there is absolutely nothing they can do to defend themselves from that. If they slack off they'll be fired, not disciplined like a normal worker just straight up fired, they simply can't afford to take the risk
Objectively, logically, the testers cannot take any portion of blame. If a player finds a bug during normal play, then a tester will find it during normal play, let alone during the mind-blowing complex test plans that most AAA developers work with.