Buying bitcoins isn't investing. Unless you're buying them to use for purchasing things, you're just speculating.
Merely buying and selling bitcoins is just gambling like a casino, stock market or lottery.
So if I buy gold to use as a store of value, and then sell it later, am I gambling? How about property as a store of value? If I exchange $USD for another currency, hold it, and then exchange it back, am I gambling?
A good reason that some sell their bitcoin back to fiat is that not all merchants except bitcoin (yet), can't pay their bills or rent with it (yet), can't pay their taxes with it (probably never will happen), can't buy a new car with it (yet), etc.
Hell, if I could use bitcoin everywhere for all my purchases, pay my taxes with it, and get paid directly in bitcoin, I'd never use $USD ever again. If that scenario came true, I would still hold some BTC in my account (like a savings account)... and that's not gambling to hold a currency.