The DC is 20 because the DM has chosen to make it 20. But in 5e, there is no absolute rule that says doors have to be DC20. If all the PCs have poor strength and the DM wants to maintain the play value of doors which are difficult for them, he can make the DC lower. He can made a dungeon where the doors are old and rotten and only DC18.
If you're playing that kind of game, then sure, but for a lot of people that sort of thing has no appeal.
It's not the job of the DM to ensure that the tasks which the PCs face are reasonable
for them. It's the job of the DM to adjudicate uncertainty; and in the role of setting-builder, it is the job of the DM to design a world; but above all, it is the job of the DM to remain fair and impartial.
The DM can build a world that has a dungeon with old and rotten doors, but they shouldn't decide that the doors are old and rotten
because the PCs have low Strength. That would be meta-gaming, on par with the random monster aiming its frost breath at the one PC who doesn't have cold resistance, when it has no way of determining such a thing.
There's no joy to be had in exploring a world that was specifically tailored
for you to explore, or at least, exploring the Matrix is not nearly as meaningful as exploring the real (in-game) world. You lose a lot in authenticity, when the DM contrives obstacles
for you; whether the DM
intends for those obstacles to be trivial or difficult, or perfectly tailored for your level, simply knowing that it was set up
for you makes it hard to care about the result. Your success isn't your own, in such a case, but a result of the DM's bias that you should or should not succeed.