#2. To advance Braun Strowman's storyline
The truth is that this storyline is likely meant to advance Braun Strowman and even McIntyre to some extent. That's actually a good thing, as a veteran like Kurt Angle will merely be a prop to help Strowman get over more.
This alliance may not be temporary. We could see the tag team feud continue for a while longer, perhaps even at Elimination Chamber, where we get a decisive finish to the match.
Charlotte Flair to return and wrestle a CURRENT CHAMPION?
#1. Too much name value to retire now
Kurt Angle is a big name and WWE will want to keep him around for as long as he's willing to go, or as far as the doctor clears him. Whether he wins or loses, Angle is a legend that can help draw old fans back to TV, and they'll want to make a big deal out of his retirement. It's understandable why WWE would hold back on it.