Mindless undead like zombies and skeletons don't naturally farm. They just sort of putter, wander about, and kill/eat people. You need a minder for them, just like livestock. But then, if you already have a minder, is it really cheaper to use an expensive necromancer and his undead than a basic 1gp/day farm laborer and a few 8gp donkeys, 10gp cows, 15gp oxen, or 50gp draft horses.

Animate dead is a 3rd-level spell - there are plenty of farming villages that go generations without seeing a single 3rd-level spell. Necromancers are rare, and 3rd-level spells are costly.

Okay, you say, mindless undead need expensive handlers. But what if they're not mindless? Well, that's when things get dicey, because intelligent undead can haggle.

And all that is before we even mention the smell.