Because if you really put the work in for just one day (mine only took 4hrs) just 3 or 4 times a year they are ridiculously easy to clean in the future. At the moment for example I could get a great finish on my car in 15-20 minutes, the dirt just washes straight off. I used to spend an hour+ every week cleaning the car, but with a few major cleans spread throughout the year the rest is easy.
I cant see why people wouldnt keep them clean to be honest. Spend £7-13k on anything and I would make damn sure I looked after it, makes sense surely? I happen to enjoy cleaning it to be honest, get outside for a few hours and get a great result at the end of it.
Same with anything else in life I guess really, I could say I dont understand why people go out and spend loads on drink every weekend, I can have a perfectly good weekend whilst sober (even out at the pub) as I dont feel the need to get drunk to enjoy myself, everyone is different!