While I cant find anything confirming why this is, its likely because its on the Reserved List, so Wizards is hesitant to errata something they cant reprint
Average mythic only $6.47
Getting benefits for opponents doing things is usually whites thing (Monologue TaxMonologue Tax, Charismatic ConquerorCharismatic Conqueror, Smothering TitheSmothering Tithe, etc.), but this is actually a callback to the red card Harsh MentorHarsh Mentor, which deals two damage to opponents for using activated abilities
It looks like a Dimir proliferate deck is being hand-delivered for some format, and Mono-Black decks arent going anywhere in all their relevant formats
Although Kaladesh was a pretty well-received set when it was in Standard, its been out of print for quite some time which can lead to the prices for singles (including Inventions) being pretty high
Splitting a case with friends can yield up to a 30% savings