Why don't airplane turbo piston engines have boost control?
The majority of piston airplane engines are based on very old technology, before computerization was available. The vast majority of airplane piston engines are carbureted with manual mixture control, prop control and turbo control and the pilot has to manage all that plus carb heat manually. 

New engine technology has been very slow to come into the piston market because certification costs are so high, and the market is so small. It would cost engine manufacturers millions to develop and certify new turbo-normalizing systems for these old engines and as there are probably only a few thousand turbo-normalized avgas piston airplanes in the world they would never recoup the costs. 

There are modernized engines on the market with electronic fuel injection, computerized engine management with turbo-control, and single-level operation. These are slowly making inroads into the market but it's going to take time.