From the mfr's perspective, there is little justification not to:
1. The exterior and non-exterior paint process are done in different stages of the body production line. The robotic paint sprayers could be calibrated with different fluid tip and pressure settings, so there is next to no possibility the different paint stages producing matching paint. Early production G70 actually suffered from this exact problem, as the bumper paint didn't match the rest of the body, because they were done in different stages, even they are the exact same paint code. If you are curious, Google why there are so many used red G70s with only a few miles on the odometer.
2. The paint process is more complicated than most folks realize. Each stage in the paint process is comprised of more than 1 coat. That means a 2-stage paint (base+clear) could be some 4-6 coats total, with flash time in between. Unlike automotive refinishing where it is just a person doing the spraying, so all he does is spray 1 coat, wait 5-10min flash time, and spray another coat. Production lines these days are fully automated with robotics, where every single move has to be programmed, calibrated, test run, evaluated, recalibrated, and re-tested. The more coats required in the overall process, the process grows more complicated geometrically. Now imagine tri-coat paints (ground+base+clear) with upwards of 10 coats, and difficult-to-calibrate ones like matte finish discussed herein.
3. Unlike exterior panels, where appearance is of paramount importance, non-exterior panels really just require good corrosion protection for the substrate. Yeah, there are guys that obsess over dressing up their engine compartment to the nines and keeping it spotless in order to show off to their buddies and on Facebook groups, the vast majority of car buyers couldn't give a rat's behind how the underhood paint don't quite match the exterior. A good portion of them probably never opened the hood.
Besides, UV protection, which is formulated into automotive clear top coats, is not required where the sun don't shine 364.5 days out of a year.