Also the benefit of time. Kuiper hasn't launched anything yet, and all their launchers are still in development - BOs own rocket New Glenn, Arianne 6, and ULA Vulcan. Vulcan probably isn't too far off, 2023, but they will need time to ramp up production. It'll also be a lot more expensive than for SpaceX.

Amazon will probably have to heavily subsidize Kuiper.

I reckon they hope the market is big enough for multiple providers.