Linux is a kernel, not an app platform. While you can use some apps across several different Linux-based OSes, the best apps are designed to integrate with a specific platform like GNOME, KDE, elementary OS, etc.
FreeDesktop.org specifications ensure some amount of cross-compatibility between Linux-based desktops, but not all specs are implemented by all desktops.
Some platforms like elementary OS AppCenter require apps to use the native toolkit for the OS.
The Linux App Summit is trying to help create and grow a broad “Linux app ecosystem.”