Why is Monstrosity not a keyword ability?
It is a lot easier to print a card like Nemesis of Mortals if Monstrosity is a keyword action rather than a keyword ability. For reference, its Monstrosity ability says this:


  {7}{G}{G}: Monstrosity 5. This ability costs {1} less to activate for each creature card in your graveyard.


Keyword abilities almost never have those kinds of riders that aren't built in to the definition of the ability.



In addition, Monstrosity as an ability would have two different parameters: the number of counters to add and the cost. That is very rare in keyword abilities and the complexity that would introduce probably factors strongly into this design choice.

Among other keyword abilities with two parameters, Awaken and Suspend are not effects that can be expressed as keyword actions, and Reinforce is specifically an action that involves discarding the card, which would be lost if it was a keyword action.