Many modern games are pretty enormous to install; often taking 5-10 gigs or more of hard drive space. In comparison, my entire / partition, including all* the software I have installed on my laptop (kernel, X, KDE, productivity software, web browsers, SQL servers, and thousands of other things), weighs in at 8.8gigs. In cases where a game absolutely requires a certain version of a library, it would be much more sensible to just statically complie them for distribution via steam. Maybe it would increase download sizes by 1%. Even if it cost an extra 10% (it wouldn't), making a consistently functional experience would be well worth it for Valve.
*except steam games, which are installed on a different partition