Value is a mix of scarcity and demand, not scarcity alone
I think you get the idea
If you are capable of healing your cards, through items, actions, or character abilities, and Beast, Tragic Hero has damage on them at the start of your turn, you can let them attack and then heal them, allowing you to still benefit from the increased strength but keeping them healthy enough to stick around
If you put in a bit of work, you could be killing every creature and gaining control of all of them
(Also see Vraska the Unseen and Battle of Witsboth legal in Standard as of this writing.) Another keyword thats taken years to develop is battalion
These decks want to be creature-heavy anyway and the cost reduction eats into the commander tax, making it an appealing counter commander for Commander Brackets 2-3