Commissions are insidious. For each transaction, they are relatively small, but after repeated buys and sells, they add up quickly. And they always come in pairs. Every time you commit to buying a stock, you will eventually have to sell it to get access to your money, whatever might be left.
If there were commissions for movie theaters, you would have to pay to get AND to get out, though it might be worth it for some films. If there were commissions for bank accounts, you would have to pay ATM fees to deposit and withdraw funds, which is likely to happen soon anyway.