Once upon a time there was an Atari game publisher. They released games for various devices, consoles and computers. One of the famous games was Test Drive Unlimited created by Eden Games in 2006. Today you can no longer buy this game in normal stores. So why isn't the game even digitally available? The servers in this game were turned off in 2012. But singleplayer can still work. I demand that Atari start selling all the games they've released. Even those released by Bandai Namco Partners.

Short answer: Car licenses.
They expire.
They are expensive.
It's financially not reasonable to spend money to renew licenses for old games.

Edit: spelling - again.

Ya, vehicle licenses are normally the main reason why a game is not re-released. An example is the game Outrun by Sega, the actual car needed to be completely redrawn in re-releases of the game to avoid paying license fees and can't see companies putting that much effort in games that have multiple licensed cars that need to be re-drawn.

Technically, Test Drive was a series published by Accolade, before going through a variety of publishers and landing at Infogrames (who then adopted the Atari name). However, Infogrames (Atari) went through bankruptcy in 2013, and in 2016 sold the rights for Test Drive to Nacon.

So, to be able to release the game here, there would be a number of steps:
1) make sure you still have the rights to the cars (these probably expired, as the other posters have stated, so you would have to negotiate with the car marques)
2) make sure you still have the rights to the music in the game (same issue as point 1, although not sure how much music TD:U had in it - might have to reinstall my disc copy and see)
3) Work out who has the rights to the game itself - also a bit of a nightmare - I mean, it could be Eden Games (who went into liquidation in 2013 before reopening, so no guarantee they still have the rights), it could be Infogrames or its successor organisations or it could be Nacon (via the sale) - or it could be an unholy mix of the three.

I'd also suggest you might want to petition the potential rights holders directly if you want them to start re-selling games; I suspect they don't frequent the GoG forum...