The exocomp basically prove Tarek71 point. When the Enterprise crew saw the exocomp they regarded it as unusual. Androids are almost completely absent, Data (and later a few others) are extremely rare.
I think one of the reasons why androids are rare is that they're not really needed all that much. With many things available at the push of a button or with a single voice command, the need for an automaton isn't all that great unless you absolutely just want to have one or are physically incapable of doing things (the majority of people in Trek don't fit that description and the few that do usually have a real person to assist them, IMO). And although we tend not to think of them as such, the various automated probes launched can be considered a type of robot. Going where it might be too dangerous or infeasible for people to go.
As for the Soong androids, they seem to be just barely within reach of Federation technology, very difficult to make work, and with a very high rate of failure. And what is the point in making them, other than as an experiment? I can see the Exocomps being useful in a niche role or two, so it might be worthwhile to make a few. Their small size makes it easy for them to work in cramped spaces. Drone ships can be described as a sort of robot
Androids, as being human equivalents in terms of function and social interaction, should be unique. Unless we have a Dune-esque prohibition against making a machine in the image of man. But, it seems that menial tasks did involve more artificial life forms, such as the exo-comps, or Mark 1 EMHs, implying that holograms were used in other jobs. It makes sense, to a degree. Also, you have the ship's computer in almost ever episode