I think they are bought with the thought that they are temperary in mind. Even worse is people who buy giant snakes with the intention of selling them when the get "too" big.
You do raise an interesting point though. Ball Pythons and Boa Constrictors are amoung the longest lived snakes and I have both. I'm 29 now and I know that I won't be keeping snakes into my 60s so what will I do with my "pets"?