Why? There is nothing stopping manufacturers from applying this to ICE cars. In some cases they already have as there are some ODB2 codes that are proprietary.

As emissions standards get tougher makers are relying on increasingly complex fuel and emission maps that are also proprietary. There is nothing stopping you from building an electric car today from Open hardware and software. The same can’t be said for ICE cars.

Well I will add some states are quite stringent with safety standards for cars made that year. So if you build a car it may not be road legal in some states. Unless your diy car meats certain safety requirements. That's why either mod or heavily modify older cars. Same applies with emissions.

Yet motorcycles are road legal. I honestly don't understand the law some times.

The workaround for this (while it lasts) is to only buy vehicles targeted at fleet purchasers.

You get the barebones version of a much smaller selection, sure, but the vehicles still tend to be serviceable by in-house/shadetree mechanics instead of the dealership.

I was thinking more a long the lines of cars without ECU's.

Most ICE cars are equipped with ECUs for various subsystems