На окошке грибы не растут

See Без труда не вытащишь и рыбку из пруда (Б)

Cf: Apples don't grow on monkey-trees (Br.). God gives (sends) every bird its food, but he does not throw it into the nest (Am.). No pains, no gains (Am., Br.). Potatoes don't grow by the side of the pot (Br.)