One thing that could help is making tagging mandatory (it’s a category-level setting) and creating some tags ahead of time so there are options to choose from. I think the most successful communities I’ve seen using tags require at least one.
I think you’re right though, as a topic author it can be easy to skip the field entirely… especially if you’re new to a community and you haven’t used tags that much previously. It’s hard to know what’s useful and you’d rather do nothing versus getting it wrong.