The whole "you build it you run it" movement is an attempt to fix dev teams just not giving a fuck about quality of code they put out, especially from a reliability point of view.

Why is the opposite better?

Probably this approach is more scalable specially in big companies where you can have operation teams on-call for a myriad of project.

I personally believe that this does not guarantee a better service.

It's exactly like properly/cleverly documenting your code/project: not only for others now or in a few year, but also for yourself later on.

It's having common rules across teams to get more reliability out of the whole company.

You build it, you run it. Fine. Until the point when you can't anymore (because... reasons - it just happens). In any activity you want to sustain, you always have to have backups (in people and in processes), instead on relying on your-(self/team) alone.

A whole business takes that into account as importantly as their disaster recovery processes (which is not necessarily something you focus early on, but you eventually do).