That's the real reason
With flying, trample, and haste, it comes out swinging right away, and when it connects, you draw cards and pile on +1/+1 counters equal to the number of different permanent types in your graveyard, turning every sacrifice into fuel for both power and card advantage
Rather, environments act as glorified menus from which you select your next opponent, but this doesnt really hamper the experience because thanks to the random nature of a card game theres more than enough variety in gameplay
That opponent is chosen as part of that additional cost
We saw a similar issue play out with Peregrin Took, which looked like a great card at first but ended up floundering in Lord of the Rings Limited
Considering all possible card types in magic at the moment (the normal card types are artifact, battle, creature, enchantment, instant, land, planeswalker, sorcery, and kindred (ex tribal)), we get the maximum hypothetical size of Tarmo equal to 9/10