Because you pay for a storefront, servers (that you need and need even more if multiplayer), advertising to a large userbase, and a lot of other services. This is basically a publishing deal on steroids, and those very regularly go above 30%.
Which isn't that bad of a requirement since it's only on base price, you can still make sales on it. Also no lockdown on out of steam microtransactions.
Greenlight doesn't exist anymore and hasn't for a while. The only requirement now is paying $100 (that they give you back after $1k of revenue), which is neither arbitrary or unfair. You could argue this lets more shovelware and scams pass through but it also allows small times devs to just ball it.
But anyways the real reason it's not on steam isn't because "it's a pain in the ass" but because of this
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Why sell at a fixed price when the industry norm for this type of game is perma in-dev patreon hell where you bill monthly.