I don’t think there’ll ever be any garages where you bring the car and tools and do it yourself; too many things to go wrong. Even supplying tools themselves, you’d need someone on hand to teach a person how to opperate the speciality tools, or even some of the basic ones, too.
If someone needed another part, and the customer needed to go get it, then that bay would be occupied for X amount of time more while the customer went to go get it, plus the waiting around for the ride to get them there. The more time it takes one person, the less you’ll be ‘turning’ the bays, thus losing money in the long run.