Why do you need siege equipment (or attacker) to attack a major settlement?

This usually isn’t a problem since some unit in an army will usually have siege attacker, but when they don’t why do you need to wait at least one turn to build siege equipment when:

Most of them can just climb the walls.

The rest of them can fly.

And most importantly, they can still break the gates without siege attacker!

Unrelated, but Tzeentchs teleport stance doesn’t prevent attrition (like it says) since it goes back to normal stance on turn end.

Not really a point to this post, just frustrated and sleepy.