I think because with these exchanges you can easily convert your altcoin to bitcoin and viceversa. The real purpose of an exchange is not to "keep" your coin for a long time (also 1-2 days are long "time") but it is to exchange your coin and after withdraw it.
An exchange is not a bank, I hope now a lot of users will start to think about this fact.
Anyway (if i's just to exchange bitcoins for altcoins), at least the people could use an exchange that suck less.