Although cursed, all these were great men with phenomenal powers and siddhis. They possessed divine astras such as the brahmastras, which no longer belonged here since the kaliyug was approaching and men with their inflated egos would have spelled doom for the Creation by deploying these astras (two brahmastras used simultaneously had the power of wiping out entire humanity). However, there was still time in the end of Creation. Hence Krishna was born with the purpose returning the divine astras and shastras to their rightful places while preserving the Creation from their effect. Krishna had to devise various ways so that no two people fire brahmastra at each other at the same time. For instance, Karna was cursed that when the time comes he would forget all that he had learnt. Thus he could not use Brahmastra when Arjun used it against him. Similarly, when Dronacharya was about to deploy it against Arjun, various Gods and Goddeses came down to request him not to. Likewise when Ashwathama and Arjun did use it against each other, both were asked to take it back.