Because it is branding and trademark and that is the way it has always been and for some people tradition counts.
That said, most paint jobs on tractors suck. Must not be very proud of their branding / trademark. NH has done relatively well but the family lines have not rubbed off at all to their brothers over at Case IH. 2007 paint, absolutely sucks, 2018 no better. When not working these tractors live inside the barn with the doors closed. I hate to think what they would look like if parked outside. Case IH use the excuse that red is the hardest color of all to prevent fading in. Look close at the brochures and get pictures when the tractor is new because it won't be like that long. I wax mine once a year with good product and they still look shabby.
My 55 year-old and 40 year-old JD's paint still looks pretty good, especially the 40 year-old.
The one that really stinks is Rhino from 2008, no prep at all, flakes off in big sheets.
If it was not such a PITA I'd strip mine down and repaint all three then clear coat them. The black engines and frames are not that bad but the red is worse than a bomb job with spray cans. The cab tractors are the hardest to repaint. The question is, why bother? Case IH isn't proud enough of their tractors to put good paint on them so why should I be?
Old Kubotas turn sort of pink, Case IH fades away, JD holds up pretty well. As for the rest I can't say. Most seem to have little pride in their products.