Because she's popular -- and/or Jeri Ryan had a three-year contract? Note that she's still listed as a regular in the credits, as to opposed to the TNG actors who are all "special guest-stars."
And, honestly, she's had plenty of good stuff to do this season. It's just that it was possibly worth taking a moment, amidst all the hoopla and sentiment about Data's return, to acknowledge that this would be no big deal to Seven, who has bigger things on her mind at the moment.
"Right. Your old android buddy is back. Moving on . . . ."
Just like the previous ep took a moment to acknowledge that Troi and Raffi had never met each other before.