I use Flatpaks mostly because I like having my base os and gui minimal as possible. Every thinking that is not core os I install as a flatpak. This is great because I didn't have to install dependencies like lib32 and other libraries on my root partition. Lean and mean.
Some are bundled, lots of the deps come from runtimes that are collections of deps. And many apps use those same runtimes. It's sorta middle point between bundling everything and sharing everything.
It’s like a separate package manager with much less granularity than most distros packages. It’s a very clever implementation that has got its pros and cons, but massive duplication of libraries isn’t one of them.