There are two places to live in anywhere in the world. Either you live in Rural area or you live in Urban area. Now when someone was living in the rural area and moved down to Urban settlement then the place he or she was living at the rural area became vacancy, so occupying one place gives room to open another place. So that of the rural settlement space can be use has farming.
I don't know of other countries, in my locality, there are enough lands to farm but nobody to farm. Specifically, my paternal home town is a ghost town now because urbanization has taken over 99% of the people. So lands full there to farm.
The space does not become vacant because in my opinion whole family does not move towards urban or rural areas. Now a days unity does not exists and everyone is in Struggle to make their own life by building their own house therefore the concept of joined family is no more exist as that was in previous age.
And one more thing is that all parts of lands are not fertilized and smooth so for making it fertilized there must of lots of expensive efforts and also some people don't allow their land for such activity because they are growing crops for their personal use only while they don't permit it for the purpose of exportation.
In previous time the people were hardworking so they work for getting opportunities to grow crops by their own but now people are more interested in getting food by using their money, previously people use power of hardwork but now people do their work with the power of money therefore the remaining land are still barren.