Well, there are other options but for some reason the market stolidly keeps on being centred around two platforms while ignoring the alternatives. For most people those alternatives - whether they be Linux distributions, browser-centred platforms like Google's Chromebook, large-screen keyboard-controlled mobile platforms like Android or iOS or otherwise - seem to be off the radar even though they often would work better than Windows or MacOS for their application.
I would argue the habits of most people are restricted to a browser on the desktop. I doubt their OS and native applications are likely to be a barrier beyond simply how their UI looks.
That may be true, but in order for them to start that browser they have to have an OS. When they go their local store and buy a random computer they're going to get Windows or MacOS if they feel like splurging.
It's a non-trivial investment to install linux or to look for a PC coming with it preinstalled, especially if it's something they don't particularly care about (since they mostly use a browser).
OSX machines are prohibitively expensive for most, so Windows is the only reasonable option.