It's not actually that hard for cross platform apps, at least. We do this in Conveyor and can build debs and apt repos, with inferred dependencies, even from Windows and macOS:

https://conveyor.hydraulic.dev/7.2/outputs/#linux

It works by downloading the Ubuntu package index and using it to look up ELF dependencies, devs can also extend the inferred list with other package names if they like. The deb installs an apt sources file and public key so users can stay entirely in the GUI. The control scripts are auto-generated but can have extra code appended to them via the config file if the developer so wishes.

It works pretty well. Conveyor is used primarily for cross-platform apps that use {Electron, JVM, Flutter, C++/Rust} and those don't normally have very complex Linux specific dependencies. Also, in practice Ubuntu and Debian and its various forks are close enough that package metadata is compatible in basically all cases.

People do ask us for Flatpak sometimes. Probably RPM is a higher priority though. Those two get most Linux users, they're stable, there are portable libraries that can make them and they can package anything without code changes. Flatpak requires mastering a new file format and the mandatory sandboxing is guaranteed to cause problems that will end up in the support queue of the deployment tool.