Because there is a push for software developers to be able to package directly for end users. Without devolving into the usual flame war of whether it's a good idea or not, once you install any piece of software you incur some security risks. It's not like distro maintainers are a 100% guarantee there won't be a backdoor in the binary, and compiling software from source doesn't free you from risks either, unless you code-review everything you install.

My point is, containerisation on Linux isn't necessarily slow—in fact it's unnoticeable if implemented correctly—and I prefer to default to having a decent amount of security by containerising as much software as I can, whatever the origin. Including, and especially software like the calculator, since it should not be able to do anything more than show a GUI and add numbers together.

In my particular case: I don't want to wait months or even years for updates to arrive in my distribution. With the Flatpak version I get updates usually on the same day they are published, since the Flatpak is also maintained by the calculator developers, and I also get a calculator which can't access my ssh keys or the internet, due to sandboxing. And in case of any breakage, I can also quickly roll back to the last version.

Without flatpak I'd need to use some rolling release distribution, where not just a few applications get updated quickly, but also the rest of the system, which I'm not interested in.