This analogy sort of falls apart in the details. Rolling your own is more akin to building your own vehicle. You might have all the right parts to have a functioning vehicle but you probably don't have the experience of a team of engineers and designers of a commercial vehicle.
In most cases you can buy a modestly sized vehicle from a manufacturer that still has a team of people ensuring the performance, utility, and safety of the vehicle.
In most cases you can buy a modestly sized vehicle from a manufacturer that still has a team of people ensuring the performance, utility, and safety of the vehicle.