Like others have said, it really is about time. I've been on a few different software projects for the past 8 years or so, and I can tell you from experience that it all comes down to time. (Also, car modeling is separate from development, for the most part.)
You want to put a livery editor in, so that involves planning it out, deciding which guys will do it, making it, testing it, testing it again, problem crops up, the guys working on it get shifted to another part of the project, more testing, maybe Kaz doesn't like the direction it's going, so they re-do it, etc, etc.
I can see that easily going on for weeks, and months, then slipping to years. Especially if it wasn't considered a top priority. And you can't always just hire a bunch of new developers to help balance the load. It takes time to spin them up and get them familiar with everything.
Plus we know how PD misses deadlines. When you miss deadlines, there are lots of meetings, there's bickering, and then priorities get shifted.
"We really want that 3D feature in Kaz, forget about the livery editor for now."
There's still no drag race feature. Something we know they've been trying to do since GT 2. That has to be easier than a livery editor. Yet it's not in the game.
Time, money, priorities.