You know I played HL1 recently. I had never finished it, so I decided to go back and finish it because I owed myself as a PC gamer. And after laughing at the graphics, I kinda got lost in the excellent level design. Umm... anyway the point of telling you all this is, is that when I finally finished it, my mind was screaming,"OH GOD WHY IS THIS SO LONG?" Not to say that I didn't enjoy it, and wasn't satisfied, but I can understand why people stopped finishing games.
I don't know why, I find games at around 8-10 hours to be the perfect length. There are some exceptions to this, like Crysis or Deus Ex, where the 12 hours felt short. But, yea, less is more, especially in shooters that just are pasting the same areas.