A Jonny
Jonny is a "rich" farmer- he owns land is worth a lot and valuable other assets. Tractors, combine, animals, barns, other equipment is valuable.
B Sid
Sid owns, a nice house, a few cars, perhaps other expensive things a stamp collection etc).
Jonny and Sid both die and their children inherit but are both subject to inheritance tax. Sid's children keep a few bits they want, they sell off the rest. Make a pretty penny, pay the inheritance tax and keep the rest of the money.
Jonny's children however can't really do that, because the farm is a whole, the assets, yes are worth a lot, maybe more that Sid's stuff, but if John's children are to keep farming - what can they sell to pay the inheritance tax?
- Sell the equipment - they can't work the fields, so the farm fails
- Sell the barns - all their crops rot before they go to market, or all the animal feed rots and the animals starve to death. The farm fails.
- Sell their animals - there's nothing to milk, collect eggs from, sheer to sell wool, slaughter to give them meat to sell.
There's no liquid assets. There's no way of paying the tax. The only way of paying the tax is to sell the farm. Then they can pay, but then they are out of work struggling to survive, without enough money to buy another farm, and there is one less farm producing food for everyone else to buy. (Assuming it isn't sold as a going concern, but it's more likely to be sold to a property developer who builds houses on it, because other farmers won't have the money to buy it.)
Bear in mind that most farmers will be making quite a lot, but their own wage won't be much. Most of their money, by far, will go into farm up keep - new seeds, vets bills, feed bills, equipment repair and replacement etc. When they die, none of that money will be clawed back.