Have you ever considered how many interior doors you have in your home? Most homes have many more interior doors than exterior, and they are not cheap, especially if you include installation in the price. While you might think that installing a door is a simple affair, the fact of the matter is that almost every single door is slightly different.
There are many factors that play a role in how well a door is installed. It starts with the doorframe. It needs to be square and plumb, or else you will have serious issues installing a door. It also needs to be firmly attached to the surrounding framework.
The hinges also need to be attached firmly. There is quite a bit of shear force when a door swings on its hinges, so having good quality screws of sufficient length is essential. You don’t want a door to shift when it swings. That could lead to it not closing right, or scratching against the floor at a certain angle.