TExt fields aren't offered because it's not going to be of any use.
The problem with a text field is that there's nothing to make the users consistent and the field is not "countable". Even if you can get your users to enter exactly the same text every time (which is unlikely), there's nothing in Jira to count it. If you did enable text fields in here, it wouldn't be much use - you are going to end up with a pie chart that has 1 slice per issue. If you extended text fields to be countable, you'd still end up with mostly one slice per issue unless you forced your users to be consistent (and frankly, if you want to force that, then just use a select list)
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