Single speeds also have the benefit of less maintenance, less weight, and less complexity.I understand simplicity being the attraction of single speed bikes to some but on the other hand if one just wants one speed just put your bike in a gear that you like and leave it there. Shifting is an option if you have the gears to do so. My wife couldn't learn to shift gears if her life depended on it so a single speed would suite her fine. No confusion which gear to ride in so no frustration.
I’ll ride my geared bike when I have to (really hilly and long rides), but single speeds are more enjoyable to me, and I ride them as much as I can. Easier for me to be “lazy” with gears, while a single speed has forced me to build strength in my core, and shrunk the size of my beer belly. Forces you to work out harder, IF that’s what you want, because you have no other choice