Why aren't the actual people involved in incidents asked to do reenactments?
That's not always the case, Many times they do use a real person to portray their own life.
Mayuri is a Telugu film based on Sudha Chandran's life and she played the role herself and it was a critically acclaimed film. She also played the main role in its Hindi remake Naache Mayuri. Also in Telugu biographical sports drama film Ashwini where Ashwini Nachappa played the role-based her own life. This was critically received well.
And as Scotty Parker said in comments Hollywood also have many such examples too:
Nine Non-Actors Who Starred As Themselves in Feature Films.
And the latest example coming to my mind is The 15:17 to Paris, where it got some criticism for doing it too:
On review aggregator Rotten Tomatoes, the film has an approval rating of 24% based on 144 reviews, and an average rating of 4.3/10. The website's critical consensus reads, "The 15:17 to Paris pays clumsily well-intentioned tribute to an act of heroism, but by casting the real-life individuals involved, director Clint Eastwood fatally undermines his own efforts. - wikipedia
So it's not always rainbow and butterflies when they cast real-life individual involved.
And also a risky move too as Tom Hank can attract more audience towards theater then real life sully. And not every real-life hero can act in films.