My guess from having formerly been married to a chemist is that machine-based chemistry is one of the most difficult areas of science. Chemistry is still equal parts art and science in that the techniques that will give accurate scientific results depend greatly on the types of things being measured for. For detecting a particular substance, there may or may not be a cheap or viable detecting device that could be built, depending on whether that thing is normally detected by chromatography, reaction with something else, spectroscopy, etc. For example, carbon monoxide detectors are great and cheap. But I think it's probably just really case by case. Chemistry is just super hard to generalize or systematize. I used to ask her why they don't have Star Trek tricorder type devices yet that can universally analyze substances, and that was more or less her answer.

Edit: for the difficulty of machine chemistry, see also the Theranos debacle.