Because 90% of the flame wars are centered around things that form only 10% of the actual distribution. And a lot of those discussions of merits and demerits are philosophical and fuzzy, rather than technical and precise.
To any reasonably experienced Linux user, it's not surprising that one distro looks, feels and works like another distribution when you look past the package manager and the system configuration tools. What matters is not how you configure your system or how you install applications but what you use it for and how productive you are with your work.
True: Installation, configuration files and choice of software vary across distributions according to the preferences of the creators. I believe that the majority of complaints about distributions are based on the "default" settings - things that can be easily changed with a few commands or a few mouse clicks. In fact, I've even seen some flame wars descend to the level of discussing the default wallpaper or desktop theme of Gnome or KDE used by the distribution - all while acknowledging that it's a matter of preference or choice! It's more common to see online discussions about boot-up times of two distributions without taking into account which daemons/services are enabled by default in which distro.
Now I admit that package/software management matters a lot, but given that a large number of Linux distributions have centered around a couple of packaging formats and their associated tools (deb and rpm) - with distros such as Slackware, Gentoo, Arch Linux being notable exceptions - the technical differences between two distributions which use similar packaging tools are almost zero. Gone also are the days when Linux distributions varied wildly in their ability to auto-detect and configure hardware and peripheral devices. The system base/core is common across all Linux distributions. True, newbie friendly distributions still hand-hold a lot more than "advanced" distributions, but then the whole point is that once the Operating System is installed and configured, the differences are minimal or negligible. For instance, LibreOffice looks feels and works the same way in all distributions.
Still, Linux + the GNU userland and assorted applications can be packaged and configured in a infinite variety of ways and it makes sense that the social and philosophical aspects matter a lot more than the technical differences. Over the years, the sheer volume of Linux distributions that have been produced make it clear that these differences matter a lot more than the technical ones. The ones who believe honestly in the technical advantages of one OS over another choose a different family of Operating Systems altogether, like the BSDs for instance. For the rest, it's all still GNU/Linux under the hood.
Ultimately the preference for a particular distribution comes down to familiarity with its particular method of system configuration and its package/software management features/quirks and almost nothing else. So it makes little sense to me, after all these years, that some Linux users feel superior to others simply because they use a "geekier" distribution and that their distribution requires the user to edit a configuration file with an editor manually rather than simply click a button in a GUI window. The "my Linux distro gives more control to the end user than your distro" is childish and nonsensical because almost every Linux distribution gives the end user the same base set of software tools, only varying in methods of configuration. When you actually think about it, unless you are really a systems level programmer with the time, knowledge and inclination to edit kernel sources, you simply do not have control over how the OS interacts with your computer hardware. You rely 100% on code written by others. You may switch a few knobs here and there, but then most distros provide those knobs albeit in differing forms.
For those of you who are interested in a deeper look at a similar topic, take a look at my article on platform irrelevance.