In Splash-managed NFL Survivor contests, the payout structure is winner-takes-all. That’s the only payout option we offer for Survivor right now, which is why you won’t see a payout customization step in the Create-a-Contest flow.
What you’re seeing is expected behavior, and you’re not missing a setting.
If you need a different payout distribution, that isn’t supported on Splash Sports at this time. You could consider other contest formats on Splash that do allow custom payouts, or continue with winner-takes-all for Survivor.