They took all the farms from the "whites" those farms still exist just under new management.
How come those farms fed everyone before and now they don't.
The land is still there, but that's only a small part of what's needed.
All the seed has already been eaten up. The farms have been stripped of anything of value - tractors, harvesters, irrigation and processing machinery - even the wiring and plumbing from the farm houses.
Of course, all the live stock is gone, along with any wild game. Even the trees are gone, cut up for firewood.
Most important, the owners are gone. They were the only ones who cared enough to put in the hard work and make something out of their farms. The government owns them now, despite what it says about returning it to its ancestral owners. Mugabe took it away from the white owners, then took it away from the squatters who thought they were the new owners. If someone was stupid enough to try and plant and cultivate it, Mugabe would show up at harvest and take that away too.
In essence, Mugabe or his cronies own them now, what's left of them. Why don't you ask him why Zimbabwe can't feed itself?
For the same reason that possession of a musical instrument does not convey make the possessor a virtuoso. The land in competent hands not only fed the nation, it generated a surplus to feed neighboring countries and created positive cash flow due to the trade. The socialists complained that the white farmers controlled all of the best land. The poor blacks couldn't compete with what remained. The blatant falsehood of that claim is now obvious.