As a mtg player, I really don't get why can't we use spin-down dice as high roll at the beginning of the game.

I'm a former competitive MtG player. My playgroup found that with a bit of practice and a specific technique, we could roll 17+ on any spindown dice almost every time (try rolling the die with it starting in your hand with the 20 facing upwards with a sideways motion). While technically you can use any method you choose to randomly determine the start of a game, these dice have been proven to not be "random" thus cannot be used in a tournament. It doesn't matter at all in casual play, but in a tournament, you're just giving your opponent a chance to play first for free.

As a fellow MTG player, mostly people hearing a story once how someone tested a spindown dice, and it wasn't fair.

But the average MTG player at a con did not take stats in college, and not worth a fight. So ya just pull out another dice and smile and roll it.

First thing I can think of is that it may matter if the distribution isn’t linear, although not really sure how it would.

The second more straightforward one is that the spindown dice vary from set to set and even within the set sometimes. As such the weighting is likely going to vary a lot.