Why do you need so many tools and inventory of parts to have a DIY home repair?

Wheels of Happiness

I have noticed that to get a repair done you need so much tools, fluids, and parts to get a repair done.   I noticed that too with mechanics.    You need hex wrench, a screw driver, pliers, cutter, oil, detergent, glue to get a repair done.   

You must also keep old parts for spare screws, washer, wires so that you can complete a repair

After a repair is completed the number of tools and stuff you employed for the repair does not show.

(Answer:   there are a number of steps you need to complete-- from removal of old parts, cleaning, deburring, sand papering, and reinstallation to get the job done.   You need a different tool for each of the step that is why)