The petrochemical and motor manufacturing industries are very powerful and influential, they work closely together and it would not be in their financial interests to promote this technology or product for the following reasons:

If you fit this system to you vehicles you will see massive benefits regarding fuel consumption and engine efficiency, because once the unit starts working it produces a gas HHO, the hydrogen gas is fed into your engine via vacuum lines and/or the air intake lines. There it is added to our Fuel/Air mixture. In the combustion chamber the pure hydrogen and oxygen give our Carbon chains a much needed boost. The enhanced combustion of the gasoline means that less gasoline is required to power the car. The cleaner burn helps remove undesirable deposits of unburned Carbon atoms. The vapourised Hydrogen and Oxygen atoms blast carbon deposits away.
The Hydrogenerator system also adds water vapour into your engine. This is not a new idea. People have been using this trick for decades. The water vapour helps to slightly cool your engine. Heat is bad for your engine. Especially in mid summer. A cooler engine runs smoother, needs fewer fluid changes, and lasts longer than an overheated one. Common sense.

"Why haven't auto makers installed these on their cars yet?"

Quite frankly it means vehicles will work more efficiently, last longer and use less fuel in the process.