TS2019 and TSW have different game engines, meaning the developers would need to build each route from scratch. Furthermore, in route development for TSW you need to take into account that the player can move around on his feet - you need to make everything more detailed, so you can't even import objects. Also, the trains would need to be remade from scratch. What you're suggesting would take around 15 - 20 years of development time due to the sheer volume of content, maybe more. And if they were to sell these products cheaper than TS2019, the company would soon go bankrupt.
Well if companies can make games like Battle Field 5, Star Wars Battle Front, then Certainly they could start working on this. It may take a while but its no that impossible. You half to start some where, right? I mean we put a man on the moon, ....so they can do it.
I have one word for you - money. Sadly, train simulation is much more niche genre than heavily marketed action games, meaning you have much smaller sale numbers and user base. (also, I don't want the "quality" of BF 5 in my train sim *cough cough*)