[2020] Why wont it start?

What happens when you try? Does it crash? Does it just load forever? Are you using any workshop content?

To see what's happening, we'll need your output log (player.log on Mac and Linux). This guide explains where to find it and also suggests some sites to upload it to, as we can't attach files here on Steam.

https://steamcommunity.com/sharedfiles/filedetails/?id=463645931&searchtext=log

It could very likely be the game is running out of memory then. There's a few things to avoid that. First I would recommend you set the max size of your pagefile to 32GB and make sure there is that much free space on the harddisk, the pagefile is on.

https://steamcommunity.com/sharedfiles/filedetails/?id=2114415052&searchtext=pagefile

That might do the trick, but you might also want to look at your workshop assets, and remove any you aren't using. Here's the average rule of thumb for the base game depending on how much RAM you have:

4GB: 0-10 assets
8GB: 100-400 assets
12GB: 400-800 assets
16GB: 800-1200 assets
24GB: 1200-2000 assets
32GB: 2000-2800 assets
64GB: 2800-6000 assets

If you have any of the major DLC's or content creator packs, reduce the asset count by another 400 each. Each DLC adds about 250-400 assets to the game.

That should hopefully give you an idea of what your system can handle. If you go over that, the game uses the pagefile as makeshift RAM, until that file reaches it max size.