You asked why, for me I'll tell ya, the voices in my head told me to.
I was recovering from my third operation in 2 years laying around outside and decided to either throw my sons old mountain bike in the dumpster that was rusting quietly in the backyard against the fence or take it and rebuild it so I could ride it for exercise. Well I rebuilt it from the ground up all parts were reused mainly the tires were shot. I cleaned and repacked all the bearing on the bike with grease mixed with royal purple 80 wt gear oil. I replaced the tires only. This bike was bought in 1989 my son had rode it out to Keyhole lake in Wyoming 3 times I think its about 40 miles to there from Gillette. I used to fly out that way to see how he and the others were doing. After we moved to Virginia the bike layed under the deck I aired up the tires one day and made a beer run where i lived in Va. it has so hilly bikes were not used by many. needless to say my beer run turned into to a beer walk what didn't get drank on the 1 mile walk got broke cause I took a gravel road short cut. The road was so steep going was easy stopping wasn't and the return was all up hill and the 6 pack was gone by the time I was back home one beer can broke open and sprayed me the rest were drank that's the last time the bike was rode. it layed rusting under the deck there until we moved to Texas and it was resting against the fence and rusting nicely when I decided to mess with it.
After replacing the tires and riding it one day i thought it needs an engine so I got on ebay and bought one the rest is history. This bike still has the same tubes that came with it 19 years old. I've changed the braking system to the side pull brakes, changed the seat and handle bars and painted it only where I've welded on it. I want to keep it ratty runs good and looks the way I want it to NASTY.
So there is my story. Of how I got started on these bikes.