Vertical integration.
They don't want to only make one piece of a machine. They want to make the entire machine.
Reason for Freightliner/Daimler powered trucks, the whole drivetrain belongs to them along with
proprietary parts/info. For several years a Freightliner couldn't be bought with a Cummins, a few
years back Freightliner allowed them back into the system.
PacCar has there own engine, Scania/Volvo has theirs, Freightliner/Daimler has their own, Mack owned
by Volvo has their own. No room for Cat and Cat isn't going to bother with that small piece of pie.
Cummins is the main free standing engine supplier, and Dodge has been a big player in that.
Emission regulations started in 1996 but development by the manufactures was kicked down the road.