Because the merit you get from aspiration and action bodhicitta is incalculably vast. Read the Diamond Sutra. The merit that comes from saving Earth creatures is very limited in comparison and the fruit is a samsaric existence. So Buddhism has a total monopoly on incalculably vast merit that comes from bodhicitta. Arhats have similarly limited compassion but they have some realization of bodhicitta, up to the Sixth Bhumi (per Lord Jigten Sumgon's teachings).
So what you are saying is that a Christian/Muslim/Jain that devotes their life to to compassionate action without a view of emptiness does not accumulate the same merit as a (supposed) BUddhist that devotes their life to to compassionate action without a view of emptiness? Coz really, bodhicitta, without overcoming the idea of subject, object and action just ain't bodhicitta. And that is what we are discussing here. A Jain, due to a belief in a truly existent self, cannot realise emptiness and thus their accumulation of merit is limited and true compasion (apparently) does not arise. I would say thatt this is 100% the same for a (purported) Buddhist that grasps to a sense of self. No difference (except, maybe, the "saving grace" of refuge).