There is no one answer Matt.
As Ian said for some people it's just the fun of the build so once that's done you might as well sell up & move onto the next project.
Other reasons;
Want something that is....insert their choice. E.g. faster, more economical, turbo'd/supercharged/V8, more doors, more space, less "boy racer", want convertible, don't want convertible, diesel, electric, etc etc.
Wife's having a baby ~ we're getting divorced ~ she/he's making them do it ~ getting a dog.
Fancy something different ~ had the car for long time ~ just not the car I thought it was ~ can't drive rwd/fwd/4wd properly?
There is something wrong with it that.... I can't afford to fix ~ won't fix ~ don't know how to fix.
Always changes car yearly.
Someone offers a price they can't refuse.
Don't like; the colour, shape, interior, manual gears, automatic box etc
Change in circumstances; lost job, got new job, company car, midlife crisis etc
Everyone's got their own reasons for doing it when they decide it's time.