Also, there are situations where you dont want to trigger Prized Amalgam, for instance, you play around Ugin minus ability or vs Anger of the Gods and you have Silversmote Ghoul/another Amalgam triggered to return at your endstep, or fetchland + Bloodghast ready to return your 3/3s on the opponents endstep
Second, Shadrix is a massive double-striking flier that can get even bigger if you choose the +1/+1 counter mode for yourself, so you can make quick work of players, whereas most group hug cards are more slow and methodical
Black also has classics like Phyrexian Rager and Dusk Legion Zealot to draw you cards, but it'll cost you some life in the process
And lets not forget that Mairsil, the Pretender gets to accumulate new abilities every time it enters the battlefield, meaning blink effects or recasting with Commander tax will continue to add more possibilities