#2. The best place to say farewell

Kurt Angle is the legend of the highest calibre, and he deserves a farewell that'll be remembered forever. Rather than abruptly ending his career, it's better that they send him off on a good note in the biggest stage possible.
Moreover, it being his farewell match instantly places it in "special attraction" territory. Now, many fans of the past as well might return to watch Kurt Angle wrestle one last time.
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#1. Vince McMahon probably forced him to

One thing is for sure, Kurt Angle is highly respected by Vince McMahon. Prior to Brock Lesnar, he was the #1 legitimate athlete in WWE, and McMahon was more than happy to welcome him back.
However, he probably respects him too much to let him continue to wrestle and hurt his body. There's no doubt that if he wants to, Angle can find a job of sorts backstage in WWE.
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