That is a very debatable assumption. The large majority of apps I use use the desktop's native theme.
I'm a dev/technical user, in the key audience for native look & feel powertools, yet the main apps I use on Mac are still Chrome, Spotify, Slack, VS Code. None of these use the desktop's native theme.
Imagine the case for non-technical users.
Average users expect apps to have a custom look and feel. Ever tried SnapChat?
> Imagine the case for non-technical users.
Most non-technical users I know - elderly couples, family, etc don't even install apps, they just use what their OS gives them. I don't know any who does not have VLC or LibreOffice however (but that may be a french thing)
The discussion here is really about average users, and the baseline assumption here is that we are talking about apps that don't come with the OS, that we'd install, since we're devs, creating new stuff.
> I don't know any who does not have VLC or LibreOffice however [...] (but that may be a french thing)
It's a French or power-user thing.
I was about to say "you know that the median mobile-device user has one third-party app installed, right?" but then I thought better and Googled it first. This used to be true, but the stat is far out-of-date; nowadays the median mobile-device user has 40-to-80 apps!
I'm kind of shocked how much user behavior has changed, honestly.
And I also thought about Snapchat as a reference for end user application.
Certainly because it's a model of simplicity and is really far from native themes.
Or because they are unaware that there are native apps available, just from third parties. For example: https://volt-app.com/
Who are 'people' here? The strawman here is that 'average users' do not expect native theme.
If you ask most people, they probably won't know to identify this as a desirable trait, but what they do know is that if they launch a non-native app it will likely not look or behave according to their expectations. For example, I'm an expert user and even I'm still tripped up by the fact that Slack has a rather anemic menubar, and Discourse's is even worse.
Windows, Linux and Android users have no such expectations. In fact, in the case of Android, a non-native look and feel could be considered a positive thing since the native android look and feel is kind of terrible.