Because it doesn't filter by platform unless you tell it to. You could have perfectly good reasons to look for games compatible with other platforms, so the client doesn't try to second-guess you.
Viewing post in Windows games on Linux
Being on a platform and look for stuff form another one (assuming you have that platform at all: not all Linux user use Windows) is the exception, it shouldn't be assumed as normality.
"you may be interested in something currently unavailable to you" doesn't sound a reasonable paradigm.
Would be this convenient also for Windows user with Linux's only games? Probably a OS they didn't even consider?
Yes, because you don't know why they're looking for games for another platform. Maybe it's for a friend. Maybe they're browsing for a mobile game, but from their desktop where it's more comfortable. You just don't know. So the app rightly shows you everything by default. Feel free to apply all the filters you want manually.