Or just tell them all to regularly communicate/listen to their team? Sounds way more efficient.

Doesn't work, because the team won't always tell you of the issues that block them (normalization of deviance). Sometimes you need to find out yourself.

You can see this when you join a (disfunctional) new team, notice the friction, then suggest improving it.

If a manager communicated with and listened to their team, but their team had written a web service with gaping security holes or disastrous data integrity practices because all their senior engineers were incompetent and/or were hired at a level that was above their ability, would that manager find out just from chatting with them?

I promise you that it's not guaranteed. You need to actually go looking through the code to find everything that's wrong.

I think you can certainly learn a lot by being curious and fishing out anything that smells, yeah.

But agreed that youll find more mistakes, if your manager also happens to be the best IC on the team.