The reason for this is that each pack type has a different set of cards, and to build a viable deck, you'll often need two of each card you want to include
Instead, it has you choose a creature type when it enters the battlefield and makes all of your slivers and nontoken creatures that creature type
They appeared in the Team Rocket expansion set for the first time back in 2000 and are specifically tied to the nefarious villains
Its not the end of the world if a card does hit (especially if its a more reactive card: reactive cards are generally safer than threats in eternal formats), and when you make nearly 200 new cards you expect some will at least intrigue Legacy players
These creatures are yours for as long as Nihiloor is in play, which gives you plenty of time to sacrifice them to prevent your opponents from regaining control
This card only sees play in Standard, and Commander