This is a great question. I doubt anyone will give you satisfactory answer you want.
For QBO actual users able to adjust column widths while using the form makes your job make much easier. All they have to do is come up with adjustable column widths on various forms that you actually use the most, that's INNOVATION. Unfortunately, Intuit's QBO is not, sorry!
Re: Why are only some column widths adjustable in various forms (i.e. bank deposits, bills, etc) ...
I understand how to adjust column widths on forms. But on other screens like journal entries, deposits, etc where I would like to widen the column width for "Class" for example, QBO only allows me to change certain column widths and not others. And when I do change the column width, it will not allow me to adjust it after I adjust the initial time.