ETS2 also has a large mod community behind it to constantly keep it fresh that I assume, SCS fully appreciates & supports. That community makes it easy to keep the game's life-support going strong when it can consistently add in trucks, trailers, real world companies, AI/Sound/Physics changes or literally new countries. I think SMS also add in their own updates/content even to date.Comparing ETS2 to a hypothetical open world race simulator feels like comparing DCS and MS Flight Simulator. They're both flight sims but they're not even slightly the same thing and I'm not sure that there's much overlap between their target markets at all.
Also, ETS2 is nearly ten years old and is regularly on sale for less than $5. Sales numbers start to mean a lot less when a game is basically being given away over such an extended period of time. Even I have a copy of ETS2, and I've played it for like an hour and a half. It's chill and I can see why people like it, but I don't find myself itching to go back and play it. Just because I own a copy doesn't mean I'd buy an open world race sim.