All PCs have hard drives, and usually the hard drives are considerably larger than console hard drives. Digital distribution is also a common way to distribute brand new top selling retail PC games. Many Steam games match the DVD size (some are over 10 GB), and are directly downloaded to your HDD. It's easier just to include an installer to the DVD version of the game that copies the game to the hard drive. Now the game can always assume it's installed to the hard drive, and the streaming technology can assume much better seek times and better bandwidth. This of course reduces the testing and fine tuning time (there's already too many different PC configurations).
Streaming from DVD has more variable performance (seek times are pretty random) compared to steady HDD performance. Many laptops also spin down the disc pretty quickly to conserve battery. If the disc is stopped, a read from it can block for several seconds. That wouldn't be much fun during a huge firefight in a competitive multiplayer game.
Of course if the game includes large compressed video files (that are not used inside the game), streaming those from the optical disc is a good choice. Optical discs have good sequential performance (video and music streaming for example), but random access performance is just bad (interactive gaming sequences have unpredictable access based on player's actions).
Also PC developers can safely allocate more memory than the system actually has. Virtual memory guarantees that allocations do not fail, as the memory area is automatically expanded to the PC hard drive. The performance of course suffers if the system has to swap a lot of data to the hard drive, but the game still works properly on all computers, even on those low end laptops. This is one reason why games that are designed solely for PC platform do not necessarily have to include any kind of HDD streaming system. But for the best performance, HDD streaming is a better bet than counting on virtual memory swapping. When you have your own streaming system integrated to the game, you have much more knowledge when to start loading the data you will need in the near future. If you count just on virtual memory swapping, there will be stalls and stuttering in the game performance, since the OS virtual memory system has no knowledge on the data you will need in the near future. With your own streaming system you can (almost) always predict right and preload everything just before you need it = fluid gameplay with no stalls and no object/texture popping.
