This seems plausible for someone with no time to go through the investment and technicalities involved in mining but then again ask yourself the same question let's say for Gold mining.
Why buy all that equipment to mine gold if one can just purchase the available goal and sell it off in the future?

1. Passion for mining/adventure
2. Possibility of hitting a jackpot. I think you have heard cases of miners ripping big.
3. Taxes? Some countries probably don't have a tax framework for mined crypto.
4. The purchased mining gear still remains your asset as it helps you to mint more coins. Can be sold off at any time to recoup some of the funds.
5. Somebody has got to do it. You can't all be traders, other people will choose to be miners, brokers etc.