Yeah, but Magic blocks is an arcade game, so your goal is to get the highest score possible and survive as long as you can.
However, in Uno for example, there is a maximum amount of points you can obtain which can easily be calculated (the value of the entire deck), and it is not possible to go above that. Not to mention that I don't even know what is the value of showing high scores in Uno, in that case that would more be like a ranking of the worst rather than best players, which already exists as a hall of shame in statistics, but it calculates the average score for all rounds.
In games where this is useful, such a ranking exists, for example Yahtzee, although display of the ranking is unfortunately currently broken, but you can still see your own rank. Unfortunately, this isn't very useful for most Playroom games, so this is probably why it hasn't been done before.
It could perhaps be done for Scrabble, especially for solo play, but other than that I think it doesn't have any value in most other games, so that's probably why it doesn't exist.