Whatever for? JR bowed out of Nemesis for that reason: Seven had no prior relationship with the TNG crew. It wouldn't have made any sense to include her at the time.
Because the story involves the Borg and Seven is a former Borg. For once, it kind of makes sense to involve her...kind of.
At the time she was introduced, The Doctor said that it was medically necessary to help her skin heal. That said, she's bared her arms on Voyager: https://twitter.com/RobertPicardo/status/1217756097731276800?s=19
Jeri Ryan is apparently only in 3 episodes, and considering the emphasis on the Borg, it makes sense to include a ex-borg character. In fairness, you don't know that the story has 'nothing to do with her' so the words 'wait and see' spring to mind.