Rather than think that it would be difficult to create a Data, we can easily postulate it would be fruitless to create a Data. What good is an android, another mouth to feed? (Yes, he does "lubricate his biofunctions", whatever those are.)
We have never gotten the impression that Starfleet would be lacking in manpower. Being a Starfleet employee is a privilege people compete for; creating employees on an assembly line would be an affront running contrary to the interests of both Starfleet and its employees.
Nor do we get the impression that Starfleet would be dissatisfied with the muscular power or computing power of the employees it does have. Starfleet personnel, like supposedly all personnel everywhere, have access to machinery that does all the real work; they themselves are there for the adventure and for the human(oid) touch. Giving the adventure and the touch over to the machines is not a goal or even an option.
Androids in the 24th century are something of a triviality, a useless curiosity that nobody bothers with. Sentient machines have their uses and their place in the society. Cramming a sentience into an android is a technological feat of sorts, which is apparently why Noonian Soong chose it as the way to demonstrate his mastery of positronics - but it's not useful, merely spectacular. And while spectacles come in all shapes and sizes, megalomania is not common in our heroes and sidekicks; creating a single perfect sentient android (while keeping decades of previous research secret because it was a hair's breadth short of perfection) is more appealing to Soong than creating 4.7 million sentient androids that would dance the perfect Swan Lake in front of the Federation Science Academy but stumble on properly smiling an embarrassed smile at the applause.
Sure, Data is unique. But that is no reason to try and have another Data. Indeed, it's a very good reason not