MTV did a little digging to find out why some games need it and some games don't and the answers may surprise you. In short, different parts of a disc move at different speeds. DVD players can read at different speeds, but Blu-ray players read at one, so DVDs being dropped onto Blu-rays causes the problems. A similar problem doesn't exist with the hard drive. There are ways around installing (obviously), but they sound like they require a bit more work for the developer.
For you PS3 owners: How much of a problem is installation? Is it a pain? Or do you welcome the return of "get pumped" time?