Spread and grow without effort, yes.... Australians chose low quality poisoned food over abundant, nutritional food that spread and grew without effort
Harvest without extensive labor, not so much.
I admit I've never tried to harvest a prickly pear but it seems the flowers and pads are the only parts eaten by people.
the fruits of are delicate, they need to be carefully harvested by hand. Then the spines rubbed off.
The pads of the plant also must be harvested by hand. Then the spines removed.