Because its custard its made with egg yolks, which are tempered in with the hot heavy cream mixture. Its then blended up with the ripe bananas, slightly brown is best, to give the ice cream that fresh delicious banana flavor.  You’ll then mix the ice cream up in your , add in the chocolate and walnuts when its almost done, just so they get folded throughout.  The ice cream will be a nice soft serve ice cream consistency. But at this point I like to freeze it up to make it scoopable.