DIGIMON First Movie

Why did Seraphimon and Magnadramon release the golden digi eggs instead of just destroying Cherubimon themselves. While in their Champion and Ultimate forms respectively they stated that they didn’t have enough strength to defeat him as is but then proceeded to Digivole to their Mega forms just to essentially do nothing. I’m not sure if the writers were implying that Magnamon and Rapidmon are stronger than 2 Mega Level Celestial DIGIMON but that seems like BS to me. What do you think? Also why did they never Mega Digivole again after that first time?