If you use the Drupal 8 'Extend' page to manage modules, and enable anything, or even just allow the default profile to turn things on for you (the usual case) the module checkboxes are disabled. You cannot turn 'optional' things off.
It's not due to it being required by a dependency or anything, the checkbox is simply locked on, with no explanation of why you can't use it. The checkbox is still there, taunting you with its permanently on status. You want to turn off "Comment" because you are not going to use it? Too bad sucker!
It turns out this is by design
The short answer is - you probably are looking for the "Uninstall" tab found on the "Extend" (Modules) page
There is no longer any grey area between a module being on and active vs off and "purged". "Disabled" is no longer a thing. Uninstalling a module means removing all traces of it.
The longer answer is because reasons
A huge debate and amount of thought went into this, with strong talk on both sides.
Reasons for this include :
* Because upgrades of some contrib module don't actually work (and can't be made to work) if upgrading when they are disabled.
* Because data left behind by a disabled module that is later re-enabled causes integrity issues and cruft.
* Some upgrades left behind data loss due to things like unexpected dependencies, plugins or hooks being unavailable at key times - when things were disabled.
.. that issue is several hundred arguments long, so It's unfair to over-summarize.
This change was made possible because the method for upgrading is now shifted into 'migration' (rather than internal upgrading) and configuration management (rather than module configs in database schema)
But things are broken - how can I remove this broken contrib module that now makes the site unusable?
... I'm still trying to figure that one out. The big discussion hints that we are expected to edit config files by hand now...
Can we make this developer experience less horrible?
I suggest we really need to replace the "disabled" checkbox UI with (at least) an explanation of why you can't disable modules, and how you can uninstall them (and what those implications are). Right now it behaves like a broken UI, a buggy form that was accidentally crippled - because giving a user a checkbox and then telling them "this isn't really something you can change" is obnoxious.