That works for typical maintenance tasks, but when you have to service suspensions, adjust bearings, true wheels, bleed brakes, replace a casette, even change cables and housing, it helps to have experience and the right tools, which bike mechanics have, but you'd need to specifically acquire if you just ride bikes.
It is worth learning how to fix a flat (you'll need to buy tire levers), clean and lube a drivetrain, replace brake pads and adjust brakes and derailers, and in general be able to turn wrenches and screwdrivers to fix things. It is also worth paying someone else to do some of that stuff if you're not interested in learning.
For more involved work you pretty much need the tools and experience of a bike shop, whether you're doing it yourself, or taking it in for service.