Simply because they have no value, and you don't sell it anywhere. Some may sell but prices are too small for us to trade. So I think it's a reluctance to keep those altcoins in my wallet.
Some times it's good to keep such shitcoins because on a long run some turn out gold coins.. Lol.. Seriously, we need to be optimistic and patient in crypto, thats the only way to succeed. A coin might be of no value today but of a great value tomorrow.