Directly from the TF2 wiki
(the official one)
Mad Milk supposedly came from cows infused with "sweet, wholesome thermonuclear radiation".
And besides, think about how much "milk" there is in a single jar of Mad Milk
Unless Mann Co. sells testicle enhancement pills, Scout would have to spend every minute of his day wanking to fill just one jar.
And Scout works all day fighting, 364 days out of the year.
Unless it's not just Scout's semen, in which case, all I have to say is Wow Scout you loving whore