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.