Because they don’t have to sell on the Mac App Store, they can use their own website. That has nothing to do with other browser engines being available.

And they still must pay the 100$ annually to sign and notarize.

Because MacOs is around 10 to 15% market share it make sense to use every incentive available to attract developers to another platform than Windows. There is a hidden technology fee, but it make more sense not to charge for it.

iOS is more like 50 to 55% market share so the goal is more to filter out bad actors and keep a high standard. Also let's milk that cash-cow.

I'm not necessarily agreeing with this approach but I got to admit it makes sense.