Because whoever owns the door isn't going out of their way to make life easy for the PCs.
Yes, that's true. That's why fail forward exists--to help minimize poor planning and design.
I'd rather instruct people on how to do and discuss better design instead of just throwing on a duct tape patch.
You missed the antecedent: the PC is chasing the assassin and wants to catch them.
No I didn't. They failed to catch the assassin but didn't somehow manage to catch them anyway. Instead they have completely different options to track him down. They failed to achieve their goal in this instance, now they have different alternatives to achieve it.
Have you mapped out the existence, location, and routine of every being in the location? Have you determined the squeakiness of every single hinge and floorboard? Probably not. So what does it actually matter if the hinge doesn't squeak on a success?
The point of the roll is "can the PCs get in?" Yes, they can--but there's a complication. Or no they can't, but they learn of another way.
Of course I haven't mapped everything. Meanwhile there will likely be other ways of getting into the house, including ones I hadn't thought of ahead of time. There's nothing wrong with "You try to pick the lock and nothing happens" because they can still achieve their goal, just not in the way they had planned. This happens pretty regularly in my games and is part of what the players enjoy.
So tell me, do you list all of these things ahead of time? If the players want to get through the door, they can: pick the lock, break the door down, find the hidden lever, knock
If so, and the players fail on all of these rolls, or simply don't think to do one, the game still comes to a screeching halt.
The game never comes to a screeching halt simply because they failed to achieve a specific goal. There are numerous opportunities and goals they can pursue instead. It's one of the main reasons I don't run linear campaigns.