From a business perspective the disk footprint is only a high cost if it results in fewer sales, which I doubt it does to any significant degree. It is wasteful, but can see why optimization efforts would get focused elsewhere.
Also the cost is often offloaded to the "greedy" Valve... So there is less pressure to optimize their own CDN use.
Given how many Steam games are bought but never even installed, it would seem not terribly worth optimizing for.
On phone, I bet you see some more effort.
It might but they have no way of measuring it so they won't take it into account.
I think certain games dont even bother to optimize the install size so that you cant fit other games on the hard drive, I think COD games are regularly hundreds of gigs