Not a supported feature. The only instance of rotated text supported in Numbers is in the horizontal axis labels of some charts.
"Numbers should also have this capability."
This is a user-to user community. Apple provides the space, but it's participation in these discussions is limited. Members of the product development teams will not see your comment here.
We who do read those comments can often tell you how to work with the software and hardware as they currently exist, but can't change the software to fit your desires.
If you want the Numbers team to consider your request, send it to them as a Feature Enhancement request, using the Provide Numbers feedback menu item in the Numbers menu.
Note that no reply should be expected. Replies are generally limited to requests for further information regarding the issue if that is needed.
Regards,
Barry