你没有注意 nothing has feathers anymore that does not fly, or isn’t descended from something that did 这部分是陈述句，句末怎么能用问号呢？ how come是非正式口语体，= how is it that ... 。例如：How come you are so late. = How is it that you are so late? Why are you so late.

词典释义：

how ˈcome (...)?

(spoken) used to say you do not understand how something can happen and would like an explanation:

‘They’ve decided not to buy the house.’ ‘How come? I thought they definitely wanted it.’

If she spent five years in Paris, how come her French is so bad?