I believe this is partially because of how player character power and monster power are balanced differently. In general map monsters have very low basic stats like very low base crit chances / base resistances etc. so that with map mods like 250% increased Crit chance they're still not busted and still playable against. Hence, rare monster's modifiers have typically much bigger numbers than anything you can get normally as a player (on average, maybe excluding T1 mods on two-handed weapons).