Because some formats like h264 are not royalty free and opera does not have the licence to use them. If you don't mind, please search my messages on what other major browsers do so as to play these formats.
For the distros you use, ubuntu builds chromium ffmpeg codecs with h264 support and opera devs make opera look for that "better version" of libffmpeg.
For arch, which does not build chromium ffmpeg codecs at all, some guy out there compiles the entire 500+mb source package of chrome and makes his own libffmpeg with support for all formats, names it opera ffmpeg codecs and puts it in his repo.
By the way, the opera ffmpeg codecs package was removed from the "blackeagle-pre-community" section of the repo, probably because it will be moved to the "herecura" one.