Its not just the replacement parts cost. Modern cars are more tightly packed and not designed for individual parts being replaced afterwards. Sometimes its just things like a screw holding something behind the center console being accessible from the engine compartment in a position that forces a full engine/transmission removal. These are tiny things but can turn a 30 minute job into two person days.

Or a oil pipework that cannot be removed from the engine because there is a bracket in the way, which can only be removed by taking out the the master brake cylinder, which requires the brake pedal to be loosened. The brake pedal braket is mounted by 4 screws, one is only accessible by removing the cladding behind the steering wheel, the steering wheel blocks that, so you have to take that off, too.

.. fixing cars is a special kind of hell.