Alduin is both a coward, as accused by Paarthunax and Ohdaving, and immensely arrogant, he's very much the sort of person to leave you in a situation that might kill you and assume it will while he leaves, ostensibly to do more important things but actually because he's afraid of direct confrontation. He leaves Sahloknir to fight you alone at Kynesgrove, leaves you in Helgen as it burns, flees from the Throat of the World when faced wth Dragonrend, and if you encounter him ressurecting other dragons will leave them to fight you alone like he did at Kynesgrove.

He presumably knows of the prophecy of his defeat, and being risk averse wants to circumvent the prophecy if he can within the limits of his own cowardice. As is often the case with prophecy stories, his attempts to head things off are part of what allow the prophecy to happen at all.