You bring up a good point that I forgot about. Some of these chemicals, like Roundup, are not just for pest control, but serve other purposes such as desiccation. Since you are going to need a chemical applicator anyway, a second machine to duplicate functionality further increases the cost ineffectiveness.
Not only that, but what I am sure most would consider the most devastating type of chemical in recent times, neonicotinoids, are applied as a seed treatment, planted with the planter. Even if you could devise an advanced machine that continually keeps pests from eating the seed while it is buried beneath the ground, that requires trips over the field that are currently not required at all. It is tough to compete with no additional trips over the field. Energy and wear is expensive and potentially devastating to the environment, including issues like carbon emissions and topsoil depletion.
(Neonics are already banned where I farm, but it is a good illustration of how chemicals can sometimes eliminate the need for machinery altogether and how the robo-pest terminator has to be able to compete with that)
