That's not true. Fortunately, we have in our society the technology (breast pumps) that allow women to save their breast milk for anyone capable of caring for the child to use at a later time: fathers, grandparents, and even the mothers themselves.
They're not the most pleasant contraptions in the world to use (so I'm told), but I wouldn't characterize their use as "very hard".
My wife was the primary caregiver for our children the first 2-3 months, and then she went back to work and I raised them as the primary caregiver. For me to provide them breastmilk after was a bit inconvenient at times, but never "very hard" (they continued to be exclusively breastfeed until ~6 months, and breastfeed in combination with other foods until ~1 year).