It can't play them because they are proprietary formats. I would LOVE it if the QT player could play WMP files, RealPlayer Files, as well as all of the .avi files. Then it would really be a godsend. Unfortunately, ass companies like Real and Microsoft and Intel won't allow their proprietary formats be used in any other player other than theirs. Really annoying, I agree.
Many of the Avi2mov converters can be found at
www.versiontracker.com . Just search for the term "divx". The best one I've found is "divX for OS X", at version 1.1.7. Pretty good all around player, and directly interfaces into the QT player so you can open .avi files in the QT player with the extra QT component. It won't work on all .avi files, though. It's probably always good to have a few different options though -- I personally have three different decoders that are QT components (/Library/Quicktime). All of them can be found at
www.versiontracker.com if you search for the term "divx". Now if only someone would make a WMP or RealPlayer file component for QT Player. That would rock.
Good luck -- hope this helps.
Oh, by the way -- does anybody else notice that the Quicktime pane of System Preferences places QT for OS X at version 5.0.5, but the QT player places both the player and QT at version 5.0.2. Anyone know which is correct?
UPDATE: You guys might want to check some of the third-party add-ons that can be downloaded via the QT updater -- just go to the QuickTime pane of the System Preferences, go to the Update tab, select "Install new 3rd-party QuickTime software", and then press the "Update Now..." button. If the QT updater says all of your components are up-to-date, click the "Custom" button, and you'll see some third-party add-ons you might as well install for better support. Select one and you'll see a description, and you can also check them to download and install them.