Zion with his injury history (and likelihood of another HUGE contract after this one) isn't going to risk getting hurt for the dunk contest just because a G-Leaguer calls him out. Why would he (or Ja for that matter)?
Endorsements? He's already got them. An easy way to lose them would be to get injured and miss a season/post-season, or worst case scenario end his career.
Money? Yeah, that 100k is nice...but minimizing your risk of unnecessary injury so you can keep signing max contracts is probably nicer.
Fame? Zion/Ja don't need more fame, they're already among the new faces of the league.
As far as risk/reward goes the chance for a player who can avg 50m+/yr going forward to injure themselves and have a setback long-term aren't worth the relatively minor accolades a dunk contest win brings.