It's two things...

1. The speed mode has an absurd skillcap, it would take you thousands of hours to actually be great at it. It's just not very straight-forward, so without seeing someone else do it, it's hard to even understand how you could improve.

2. On a Korean stream they showed that countless other characters, character skins, and carts already exist (tons more tracks too). These are meant to exist in some kind of progression/monetization system... notice the colored rarities on each of the items, and the exp leveling system. The original game had a system to unlock better licenses that would grant you access to faster and faster races, which change the optimal lines/strategies on each map with each tier.

So yes it's a simple game, but it has incentive for hardcore players in it's technical difficulty, and will later have incentive for casual players in earning upgrades and cosmetics. The current build is basically just for crowd-sourced bug/server testing, not for comments on the progression or the breadth of the game's content.