As an aside, my favorite variant of this is from an old game Puzzle Pirates: https://www.youtube.com/watch?v=XRQIHyC__kc
Which proves it's not gravity -- gravity would move the same speed always (assuming a vacuum game stage), or would move proportionally to air resistance (so based on shape), etc.
Clearly the real answer is that Tetris is played in a weightless vacuum, and that pieces are shot at a constant speed from somewhere offscreen.
Obviously not, since the speed increases as the game progresses (of course the planet could have increased its mass in the meantime).
Obviously the tetrominoes are being fired by a variable speed railgun and the current is being increased with each round.
> Board size - gravity rules - scoring etc. are arbitrary
So 'gravity rules' refer to falling speed, which is unimportant and doesn't fundamentally change the game, same with board size.
There's dozens of Tetris variants, all with tiny little differences at this level, and they all contribute to different strategies.