You're missing the intent of the number of choices.
Sure there are hundreds of different distros out there, but most of them are special-purpose. ie DVR, database, gaming, etc.
There are much fewer GP distros--Fedora, Ubuntu, and Slackware--along with their derivatives.
Compare that choice to what MS gives us--Personal, Home, Business and Enterprise (or whatever they're naming them these days). All of those actually have the software on the disc--they're just not "turned on." With Linux, everything's turned on by default.
BTW--file extensions mean nothing in Linux. Literally.