The problems in building profiles are many. The fact that every product treats the camera like a scanner is just one of many reasons this doesn’t work very well, with virtually any off the shelf product a user would have to shell out money for.
The other reason is, it's mostly not needed! It's a problem in search of a solution. In the years that ACR has been around, I'm pretty convinced that the lack of a means to use a custom profile hasn't been an issue whatsoever. Since LR uses the same engine, I don't suspect nor see a reason to revamp the processing engine Thomas designed using his mix of 'input' profiles. But should any user feel the desire to spend the time and money to build profiles, there are plenty of other solutions on the market that will let you play in that sandbox.