The explanation I've consistently heard is because it's supply that was meant for restaurants and the like, as Frumple mentioned, and that in the process of re-arranging logistics to get that product out to people directly, those businesses would lose money. It is cheaper to deliberately waste it than it is to get resources to people who need it. And we're not talking just milk. All kinds of produce. Fruits, vegetables, eggs, etc.
But the problem isn't just supply chain disruption. It's also tens of millions of people becoming unemployed. Even if all of that wasted food were immediately re-routed to grocery stores without a hiccup, many of those unemployed would still be unable to buy it anyway, and it would go to waste all the same. Which is what has always happened all the time, it's just happening on a larger scale right now.
And even one step further - the capitalist structure creates this situation where simply giving stuff away is something special that has to be bureaucratized through charity organizations in order to co-exist and reconcile with the imaginary number game. So food banks are limited by personnel and funding in their ability to absorb and distribute that food, which is something that's also actually happening right now.
But if simply producing stuff and making it available to a community unconditionally without the intervention of an imaginary number game were the norm, there would be absolutely no problems right now. Simple fact is, with the exception of inner city poor, everyone else who is starving right now is doing so within trivial distance of food that is being deliberately wasted. It wouldn't take an immense effort to solve that problem. And the only reason it's not is because of the influence of the ultra-rich. Because the exchange of goods has to be reconciled with a system that operates to their benefit. They own the companies that own the people who are producing that food, they own the legal system that governs how charity is allowed to happen, etc. It all has to operate for their profit or it doesn't operate at all. And when a pandemic forces economic activity to stop, that means it doesn't operate at all.
This is the reason for the massive push to re-open business despite the pandemic still raging, btw. Because the upper class faces a choice. Either face popular rage because millions go starving and homeless at the same time as there is obviously food and shelter to go around, on a much larger scale than the existing propaganda machine can mitigate. Or distribute resources, and by extension their own wealth, freely to prevent an angry breakdown of society during the pandemic, show the people that all the decades of claims that this was impossible was a lie all along, and expect that they'll never want to go back. Or press all their leverage into forcing a return to normalcy, with the only difference being that the gears of the economy require greasing with more blood than usual to keep turning. You can see which choice they're going with.