I’m thinking this comes from the consideration of taking imagery at the root of people’s brains when they hear Linux. Reiterating elements of the Windows or Mac UI over the decades, even if they had small visual changes, enable a significantly large population of the world to imagine the desktop even just while mentioned in a passing. Anyone that doesn’t use either of these OSes at least can have a basic imagery popping up about it due to constant advertising of the desktop via direct ads, support pages, tech websites using generic desktop images, screen shares, etc.
Linux is wild west in this regard. Everyone knows how Windows or MacOS looks like thanks to their abundant copies of descriptive bounty posters, but only other Linux users are familiar with other Linux desktops and that is usually as the names of fellow bounty hunters.
Switched from the default win10 mail app to thunderbird about a year ago when the mail app started forcibly updating to the outlook and broke some shit on my windows installation to use a whole lot of resources. I quite liked the old mail app of the windows, but Thunderbird is quite enough of a replacement at default settings and much more customizable after fiddling. K9 has no difference than Gmail on default settings, either.