Because I can't beat the game. Every time I enter the "We're Not Alone" chapter, the game freezes up when I try to open up the first door I see. I keep going back to earlier saved games to see if maybe it was a problem with the current autosave that happens a second before this problem, but the same thing happens every time. I could start a new game and play all the way back to where I'm stuck, but I'm not in the mood to replay the game for 2 hours right away...

Edit: I did some quick snooping around the web and found that I'm not the only one experiencing this problem. Apparently, the last Half-Life update screwed up Opposing Force.