Because the jobs don't need to be done at the same time or by the same people. Specialization is a very useful way to get something faster better AND cheaper.

Oil changes places are setup to make oil changes fast and easy: there is a pit in the floor, and someone in there at all times. When you pull for your oil change he has the right wrench on your engine almost before you turn it off, and is only waiting for "go" to start letting the oil out. The guy on top is meanwhile looking up the oil filter (which they have in stock: they turn inventory fast enough that accountants don't worry about this cost) and how much oil to put in. When the oil plug is replaced below the guy on top has a machine programmed to put the right amount of the right oil in. The result is they can do your oil change in 10 minutes with 2 low skill employees and they make less mistakes since the machines are programmed to prevent them.

Compare to your mechanic: he needs to jack your car up (he has a power lift not a pit), then move the drain pan into place. Then one the lower end is done jack it down, call the warehouse to deliver the right filter for your car, then look up how much oil to put in and open the cans and count. In the end your mechanic ends up spending more man hours than the oil change place and is more likely to make a mistake because less is automated. Your mechanic is also more expensive because he knows how to do other work and has tools for that, you pay for all those tools and knowledge even if you don't use it.

That pit in the oil change place is very dangerous (exhaust buildup, people falling in), engineers have spent significant effort figuring out how to make then safe. That pit is also expensive, if use it for 30 oil changes a day, 6 days a week, for 10 years the cost is pretty cheap per use. If you are a mechanic working on general repair not only is a lift cheaper and safer, the ability to position the car at any height make it more useful: if you have a both you will often choose the lift making the cost of the pit something that is amortized over less uses.

Similar applies to tire changes: they need some expensive specialized equipment that your mechanic isn't using but still needs to pay for. The tire change place has more tire customers to amortize this with.

Note that the above specialization works because oil changes are tire changes are common enough that it is worth the effort to make it work. Your town doesn't need to be very big for there to be enough cars to support many specialized oil change and tire shops. This quantity of shops means that engineers who design specialized tools have a large number of potential customers to split the R&D cost between making the costs lower. Tires and oil are all similar enough between cars that one machine can handle most.

As we get out of the common repairs we get to one off: most engines don't need a head gasket change anymore, for 200 million dollars I can design a special tool to do the job much faster: labor might only be $20, but your share of the tool R&D is $1 million dollars since only 100 people will replace the head gasket for that type of engine. Or you can pay a mechanic to do it without the tool, your labor is $800 but not special machine to pay for. Of course if your engine turns out to have a common problem things will change: mechanics will develop tricks to do the job faster, maybe some tools will be made, but it will probably never get as cheap as the machine I envisioned.