You drive a car because flying everywhere is expensive and not possible in most cases. You can't fly to the grocery store, to work, to school, etc. This isn't a very good argument. A better analogy is that you trust yourself to do car work better than you trust a mechanic. They are the expert and cost more to do the work but you have to read up on how to fix things and spend your time doing the work yourself. The expert costs money, you cost time (which is also money). Now your engine needs fixing. Do you pay for someone to figure it out for you or do you try to do it yourself? Both choices might have disastrous results; you might fuck something up big time and have to spend way more time fixing your mistakes, the mechanic might not get things done on time or within the estimate he gave you. Hell, in both cases, you might have your car stereo stolen either right from your driveway (a targeted attack) or from the mechanic's parking lot (an attacker looking for the easiest thing to steal). You might have a motion light pointing at your driveway and yard but its just you at your house guarding your stereo, maybe you spot the guy because he just walks right up to the house and the light goes off. The mechanic might have a fenced-in parking lot with a rent-a-cop doing their nightly drive-bys at various businesses, maybe he spots the guy trying to get past the fence but failing.
It's all a matter of money and time. What do you have the most of?