Traditional vendors are not willing to give the general public those kind of improvements not because they can't, but because they feel it will hurt their bottom line by not being able to sell more new car. Customer are left on their own - and so they resort to patch their car with parts that drop their warranty - sometimes for good reason, sometimes not.
For example some vendors are famous for selling the same engine at different price point, throttling the low end - a bit like what's done on some CPU/GPU in the computer world !
I put more stock in the idea that traditional vendors are terrified of any kind of pathway for frequent/easy access to the onboard engine computers. Just look at the kerflufle around stuck gas pedals; imagine what the public will do to the first vendor whose ECM is maliciously modified OTA, or who pushes an update with an unnoticed key regression or new bug (hardly an unusual thing in the world of frequent updates)
Actually, some other manufacturers do provide such free improvements, though they would require a visit to the dealership. McLaren provided performance improvements for the 12C, even after the model got discontinued.
Many manufacturers do actually sell you these upgrades. Mercedes has AMG chip tuning, Volvo calls their stuff Polestar (swedes and their funny english), BMW does it at least for the 1-series, etc etc. In most cases it includes not only ECU remapping but also cooling upgrades.
As for throttling the engine to sell at a different price point: in many countries this relates to government taxation on horsepowers. I don't see this as particularly nefarious by the car manufacturers.
"Traditional vendors" routinely update their cars to give better performance where it doesn't cost them anything and brings other benefits e.g. safety. Especially if the components are reused during multiple generations e.g. MY14/MY15.
I am buying a used LandRover now where I will get more than a 0.1s performance improvement when I take it into the dealership for a transmission software update.
