

It doesn’t have to be, but if all Linux apps were standalone binaries, installing apps would be a PITA. Flatpaks have better integration with the desktop environment (like automatically handling desktop shortcuts), can share runtimes to save space, have a standardized way of handling permissions and launch options, etc. The Linux world is moving towards flatpaks for many reasons, and the Tor Browser flatpak is marked as official from the Tor Project. Wouldn’t it be reasonable to expect it to work, and to get some sort of notification if it breaks?
Then it’s not clear what you were trying to say. Does it have to be a flatpak? No. It also doesn’t have to be a standalone binary. It’s up to the Tor Project how they want to release it.