Sorry about the but this is a about why this keeps happening. (And not just because I'm guilty this cycle.)

Because there's a real reason behind this is the network TV premise/procedural pilot. https://twitter.com/moryan/status/1352864373136633856

The first episode of SVU doesn't start with them not SVUing. They are already there and the procedure teaches you who they are & how the show works. A premise pilot is a pilot where the premise of the show happens in front of you. The Vet becomes a blackjack dealer! Or whatever.

Royal Pains is a premise/procedural pilot. Case of the week, but the first episode is how he got there (lost a patient, hid out in Hamptons.)

Problem is, networks want the audience to know very quickly (in the teaser) that they are not in for serialized story like This is Us

So you have to deliver - in the teaser - a taste of what the show will be every week. When faced with "show us that it's about catching bad guys in the teaser" AND "make sure you show us how he got there in the 1st act" - we use this structure.

It isn't because we all suffered the same writer brain injury. It's because networks want to see a character who hasn't yet found themselves in their job for the show, in their job for the show before they get there.

Is it because they don't trust audiences? Sorta.

Anyway, Mo is right, we should always interrogate the tropes. But also... don't abandon them to abandon them. Understand why they exist and figure out how to use/change/improve them.