Also, Gamers actively get angry with developers now if a game releases on something other than Steam, so it's probably better for PR and adoption rates to just target Steam and be done with it.
Installing a game standalone doesn't annoy anyone I know (though, to be fair, I'm now older with few friends)
"Game manager" is a especially visible case because, with the exception of Steam, Battle.Net, they're almost universally crap. Some of them are so bad I wouldn't want to install one even if it was the only game platform in existence!
Every Ubisoft game since Black Flag that I've bought from Steam has required Uplay.
I'm sure there are other examples, but Steam isn't preventing users from being hit with another "game manager" download. In some cases, it facilitates it.