Because it is the default desktop for at least the three biggest Linux distributions.

Why is it the default? Because GNOME was GPL when KDE depended on the non-free Qt, a long time ago. That is no longer relevant but it was the basis for it being blessed as default.

I too use GNOME, not because I particularly like it, but because Linux lets me be productive right out of the box with a minimum of fuss and therefore I want to stay close to default. But I dread every major update where I have google some obscure configuration options just to leave my productivity alone.

See my earlier point. Momentum.

They used to do something well and they're riding the momentum, deceptively, and they happen to be doing it during a time where less-techy folk and business are beginning to more see the value.

By that logic, Windows is the best operating system. After all, there are millions of PCs out there running Windows, and the vast majority of those people don't switch.

Yep. In fact, less developers are using GTK (and migrating their projects to Qt[0]), and less users are using GNOME, over time due to its self-imposed limitations. KDE usage, in what metrics I've seen, is somewhere at parity with GNOME for DE market share and growing while GNOME is declining, which is telling given that KDE is not the default DE in major distros.

Synthetic "popularity by default" vs. authentic popularity at work.

0: https://en.wikipedia.org/wiki/GTK#Criticism