Automation still needs to be cheaper for it to take off.
You're talking about building a fleet of robots (a fairly large fleet per operator, considering the size of agricultural plots in some parts of the world) that need to operate in harsh conditions ranging from near-zero temperatures to upwards of 40 degrees C, in rain/sleet/snow/sunshine, be impervious to high-wind blown dust, and generally be rugged enough to do farm work. They have to be cheape enough to be replaceable when a unit fails, or easily repaired on site - which will have to be by trained staff with some specialty knowledge.
Not saying it won't be possible one day, just that there are a lot of economic and feasibility hurdles that have to be solved first, and convincing farmers that this is better is going to take some time. You've had to prove that the costs of purchase, operation, maintenance (human salaries and parts), and replacement total to a lower dollar figure than traditional farming, and the chances of that being done on a widespread basis inside a decade - or even four - are slim.
Factory farms might do it first, for obvious reasons, but a lot of crops are still grown outside a factory setting by individuals or small corporations spread out over wide areas.