Why are altcoin exchanged for btc and not for dollar?
having tons of altcoin is just like having bitcoin. If bitcoin grows, they grow. If bitcoin falls, they fall.
Isn't this pointless? Why not buying and selling alts for dollar?
Because the cryptocurrency was created so that ordinary people could fully control their funds and not depend on the banking system and the state. As the dollar is the basic unit of exchange for fiat money, Bitcoin is the main unit of exchange for altcoins. The point of cryptocurrency is not that it can be bought for fiat, but that you can use it as a means of payment anywhere in the world and you do not need to convert it to another currency.