> Why wouldn’t you put your inputs in a form element?

Not all inputs are form-submission data.

For example a datalist-backed input to scroll to a specific page/chapter/section/subsection in a long document. You might populate the datalist with hundreds of entries so you don't have a long list of links that the user will have to scroll through in the sidebar. You can perform the scroll on the change event of the input.

That's a good UI for the user, instead of presenting a long list of links for the user to browse/search through, they simply have the input auto-suggest based on the populated datalist.

I mainly work on a large complex SPA with UX that does not often look like a form, but does have lots of inputs. These days I'm much bigger on semantic HTML, to the small extent it matters in our case, but there is a large burden of pages which were just div soup and loose inputs.