Because you're providing entertainment for a group of players who've taken time out of their schedules to play a game of heroic adventure. The occasional setback is fine, but when it's time and again "you can't complete this mission" you get players who are frustrated, who might begin thinking their hobby time would be better spent playing a video game.
The feel of D&D is to be able to delve into tombs, recover mysterious treasures, vanquish evil. It's not supposed to be a regular occurrence of "let's chalk up the last session or two as a learning experiment and come back in a few levels, spend another couple sessions getting back to this same exact spot and then deal with this low-level threat that has been inexplicably gated off."
I've bolded a couple of assumptions you're IMO wrongly baking in here. Not everyone plays for heroism, or to vanquish evil; oftentimes those outcomes are merely side effects - perhaps not even intentional - of the adventuring the game is truly about.
Further, a video game where you never fail is also going to become mighty boring in a hurry.
Here, if there's a door they can't pass then so be it; they can't go that way so instead they might as well go where they can, loot what they can, and make a note to maybe come back later with better door-removal tools...or leave it for someone else to do.
The last adventure I ran, the party had to bail back to town something like six different times in order to regroup and - often - recruit replacements for the characters that had died. That said, there weren't any hard-stop elements in that adventure such as the unopenable door example, just more opposition and danger than the party could handle on one run. Certainly a sense of accomplishment when they finally did finish it.
Starting with 4e, however, the concept of having to take more than one run at an adventure seems to have been largely pushed aside because having to retreat and try again is frustrating for the players.
You say frustration drives players away, and that's fine with me: a player who can't or won't handle some in-game frustration isn't someone I want at my table, nor is a player who has been conditioned to expect to be able to curb-stomp or immediately solve/bypass anything encountered.