Well, IMO this is just a blip in history, a brief period where some developers thought they'd flirt with pushing PC tech and seeing what happens. Traditionally, as Joker has already stated, PC developers just went with a common denominator, and any use of higher end, contemporary hardware was restricted to a few token guestures like slightly better shadows or reflections or such.
A common denominator? Sure. They weren't all targeting the highest-end card all the time. I don't think anyone would say they were and I don't think that anyone's saying that PC games should always target the latest hardware. It's just... how old is DX10? How many DX10 games are there?
Because games certainly never targeted the lowest common denominator. They still don't -- the lowest common denominator is all the people with IGP graphics! They did aim higher, though.