That's the point of flatpaks - you don't.

You build one flatpak and it will work for all distributions.

History has shown that distro maintainers aren't perfect at patching security vulnerabilities either and that sandboxing is useful regardless. It also shows that user want working software and will go through the effort of inventing new package formats like flatpak to work around distro maintainers. Maintainers now have a choice between complaining that everyone else is doing it wrong and eventually becoming irrelevant, or getting with the program and maybe even offering their expertise to accomplish what people want to do

Why would you use software if you think the dev is too incompetent to package it?

How often do distribution maintainers actually audit the package source code?

The discussion is mostly about what a distribution should contain. I don't think a distribution has to contain all the possible software applications in existence.

Instead, I think distros have to provide the base packages like desktop environments and related software. All configured for compatibility and complying with the distro philosophy.

But third party desktop applications that are not directly related to the desktop environment are a different category. There is an endless amount of them with varying quality and resources. You cannot expect distro maintainers to spend time on all these random applications.

However, if a third party app is not included in a distro, it does not mean users have to build the software by themselves. That is the problem that Flatpak and Snap and others are trying to solve. They provide sets of distro-agnostic libraries that developers can target instead of having to target each distro separately.

This way a developer can only package the app once, distro maintainers don't have to do extra work, and users can install applications without having to manually configure and build them. Everyone is happy.