Because most of them have no idea just how grim the life of a subsistence peasant farmer really is, and how that differs from the lucky few who get to sell boutique "organic" produce to rich people.
You could say the same about oil rig workers or coal miners and they are going to use both coal and oil when they grow up. Should we teach kids how to do these professions too? Where should we stop? There's many "grim" professions, things that need to be done in life, yet school kids are usually shown none of them. So what?
I don't think people should have to be subsistence peasant farmers.
Farmers in developed western countries probably have it worse. Supermarkets have driven prices down to ridiculous levels, so the profit of, for example, dairy farmers is very low. There are reasons the suicide rates in rural areas is high.
Economists tell us the "developing countries" will never actually develop until rich countries stop subsidizing their agriculture. Those folks in developing countries are stuck.