[everforward]

It's mostly about user intent. If you install the LXQT binaries on Ubuntu, you probably intend to use LXQT as your DE. It makes sense to set that up for you.

There's also an accessibility aspect. Novice users are likely to struggle with setting up the configs, so auto-configuring the packages makes them more accessible to novices. Wanting to install a package without configuring it is a poweruser kind of request; it makes sense to require poweruser knowledge to get a poweruser install, rather than making poweruser-style installs the default.

Fwiw, I believe most package managers can be told to not run their configuration steps if you really want that.