It is a good point really, why bother to augment manpower when manpower is cheap and plentiful.
A couple of theories, all keeping in mind that "rule of cool" likely has something to do with it.
1. Servitors are valuable as a form of deterrence for criminals and dissidents, being turned into a lobotomized, cyborged slave is a fairly gruesome fate and one which encourages the masses to toe the line and underlines the futility of not serving the Imperium.
2. Servitors seem to last longer than conventional workers, given that skilled workers are presumably not as common the use of "mono-task" servitors who provide a dedicated service in a relatively high skill area that one wants to maintain such as pilot servitors, gun servitors, the ubiquitous servitors slaved into warship bridge stations or whatnot. Training a pilot and hanging onto one, especially when by definition they operate a means of escape, may not be as desirable as simply ensuring loyalty and long term functionality.
3. An extension of the above, if servitors can be programmed, an idea supported by some of the source material, then high skill servitors might in fact be faster to produce and cheaper to produce as well as cheaper to prodcue and longer lasting than conventional specialists in their area.
4. For low skill heavy lifting, an obvious candidate for mass labour instead, simple servitors aren't necessarily replacing workers so much as workers and machinery. Cyborg technology is clearly prevalent in 40k and it is possible that the economics of it make it cost effective over maintaining wage workers and heavy lifting equipment.
5. Certainly if you are also interested in loyalty, dependability, deniability, limiting witnesses, etc. they have a certain niche role. Note that in confined and controlled environments like the mines of sepheris secundus or the lower decks of warships vast quantities of fuctional slaves, either serfs or press-ganged ratings, fulfill many such tasks because the above considerations are largely irrelevant. If anything, servitors are more likely to be present in sensitive areas. The high incidence in literature directly corresponds with the fact that most literature focuses on more significant players, and the endless impoverished masses that make up the majority of the human population in 40k may have considerably less contact with servitors than we do as "observers."
6. The relatively irreversible and specialized modifications conducted on amny servitors may make them less than useful or functional in the rest of their lives and be reserved for servitors as a practicality. Why go to the trouble of modifying workers who try to move on, take a better job, work inconstantly, want spare time, need sleep, or commit suicide? Lobotomize and forget.