You will find that even in many tunes made with programs that support it.
- Laziness,
- the possibility to quickly undo it (in programs where resizing cannot be undone),
- experimentation,
- importing a file previously made in a format that doesn't support variable pattern lengths such as MOD/S3M,
- simply being used to it or
- simply the lack of knowledge of such a function or simply being used to doing it this way
may be reasons for it.
Besides, I'd hardly say that I've seen this in "every" XM/IT module, not even a considerable amount. I've seen it in a few tunes but there are way more modules that actually make use of variable pattern size.