SimonG: Actually, when you buy a game on Steam, they are your contracter partner in the sale. Making them responsible for the game to run.
The sale of a subscription that grants you access to download the game, yes. Remember that Steam doesn't sell games. only the licenses necessary to access them. Anything beyond actually downloading the game is completely extra.
So if something went wrong in the system in regards to your acquisition of that license, or a rogue Valve employee voided the license or somesuch; then and only then would Valve be responsible for the game not working. But if the game simply doesn't work then they're not obliged to care, since they've only sold you a pass that allows you to download it. If you also want to play what you downloaded, then that's extra.
That's not to say that Steam support won't help you in most cases - they're just not obliged to; like Amazon's refunds. Amazon
shouldn't refund you in 99% of the cases that they do, but they're nice people. Ultimately Steam is just there to
grant you access to the games that your licenses permits you to access. It doesn't really care (but will do sometimes) about anything else.