[ranger_danger]

Firejail on Linux and Sandboxie on Windows means I can let programs install whenever they want, and I always know it'll be inside some other top-level directory I specify.

Probably better to not let programs spray all over your filesystem anyway.

[bigbuppo]

Ah yes, because people should be forced to use some random third-party program because developers went out of their way to write their own installer that lacks a feature every standard installer has had since the 1980s.

[zahlman]

Making the project work when it's in an arbitrary place isn't always trivial.

The proposed "random third-party program" is effectively an extension to the OS.