Athletes engage in athletics. Thus their “greatness” is necessarily dependent or contingent upon engagement in the activity in question.
The MGB isn’t about doing something and being “great” at it, the MGB is about being: Maximally Great Being.
Now most beings are contingent. Their essence is distinct from their existence. Or, put another way, the fact THAT they are is inherently separate or distinct from the question of WHAT they are. Any greatness exhibited is, therefore, contingent upon the fact of existence. There is no necessary greatness to be grounded in their being because their being or existence is accidental and not integral.
The ontological argument isn’t about activities engaged in by beings, nor is it about contingent beings or contingent properties of contingent beings precisely because the properties of contingent beings are, in themselves, contingent and so is any possible “greatness.”
So, no matter how great a contingent being is, that greatness is separable from its existence and therefore its greatness isn’t inherently attributable to it because its existence is incidental. There is no essential greatness to be found in contingent beings because their existence itself is contingent.
This is from Gottfreid Leibniz…
It is farther true that in God there is not only the source of existences but also that of essences, in so far as they are real, that is to say, the source of what is real in the possible. For the understanding of God is the region of eternal truths or of the ideas on which they depend, and without Him there would be nothing real in the possibilities of things, and not only would there be nothing in existence, but nothing would even be possible…
For if there is a reality in essences or possibilities, or rather in eternal truths, this reality must needs be founded in something existing and actual, and consequently in the existence of the necessary Being, in whom essence involves existence, or in whom to be possible is to be actual. (Monadology 43-44)
Ergo, all greatness derives from the potential for existence. Things that don’t exist CANNOT incorporate any great-making properties. Thus, contingent existents aren’t great in themselves or by virtue of what they are because their greatness is contingent not upon themselves but on something else. The maximal possible greatness would only be possible in that Being whose very essence is to exist as the source of all greatness, since all greatness found anywhere would be necessitated by the fact that the MGB exists necessarily, but that could only be true if the essence of the MGB (all the great-making properties that define what it is) necessitate its existence (that it is.)