Why buy a Trek? I don't own a Trek, but I think I can imagine an answer or two:

1. They are the primary brand my LBS sells.
2. They come in a nice blue color and my favorite color is blue.
3. Lance rides one, so they must be the best (especially in black/yellow).
4. They are a "known" brand, so they are a safe investment.
5. My last four bikes were Treks.
6. The guy across the street rides a Trek.