If I have 100 dollars in an investmant company. And the stock market is good and I have 120 dollars in my investament account. Why can't I just take out the 20 dollars instead of having to sell stock. Thanks

Because you don't have any money in that account... just the stock... which went up in value after you bought it... to get money you have to sell some of it.

ergo.. you bought one share in Turnip, Inc for $100, today its worth $120... you still have just one share of stock. No actual cash.