However they have many disadvantages:
Cost - $500 worth of fuel for the tractor vs $250 for chemical + $50 for fuel to run the sprayer is the big one (These cost numbers are what I've heard but I might remember wrong).
CO2: that $500 worth of fuel vs $50 directly translates into CO2, which in turns translates into global warming. (this is the only one that you can argue doesn't have a financial impact on the farmer - and it isn't clear that those arguments are valid)
Soil disturbance. Every time you til the soil you disturb the natural soil. It isn't as bad as deep plowing, but it is still harmful.
Soil compaction. A repetitively light sprayer can cover 100+ feet of crop in just two tire tracks (the first wheel the runs over the ground does the damage, the second barely adds anything so the 4 wheels result in two tracks). A tractor is heavier (the impact of weight it not linear though so figure 10% worse), and can only do 30 feet so you end up with just over 3x as many tire tracks compacting and harming the ground.
Labor cost. Sprayers can run at 15mph in the field. Cultivators are probably running at 5mph. Not only are you taking 3x as many passes, each pass takes 3x as long. You pay the operator by the hour.
Now for your robot which applies? The weed pulling robot will only disturb the soil where the weed is. However the robot needs to get to the weed which increases compaction along each row.
My guess is farmers will spray everything as a primary prevention. Then they will fly drones over the field and send a robot down only the rows where there are weeds. That is just a guess though, you can imagine your own parameters for future robots which will change everything.
Note, herbicides are not nearly as dangerous as popular press makes them out to be.
You must not be familiar with modern farming. Most farmers do no till these days. They never plow - ever. The first seven years of no till they do see less yields than their counterparts that plow, but after that the advantage of non-distributed soil mean they get better yields. (more profit ever before we account for the fact that plowing uses a lot of expensive fuel)
Nothing is simple, but there is a reason most farms and all the big ones are no or limited till.
That may be a use case, but the large majority of grown food is not organic-certified. Does it make sense for such productions?
No plowing, no pesticides, just preventive coverage and good tools.
In the long run, those large monocultures don't make sense anyway.
Robots can be great for a lot of high maintenance, Human intensive, high value cultures such as fruits, greens ...
I guess they could also help expand vegetables farms to non cultivated areas, in some autonomous ways.
For large surfaces would that financially make any sense though?