That sort of library is basically what the industry's vast catalog of integrated circuits amounts to, except that it is not open source and generally at a higher level of abstraction than the subcircuits you describe.
Really? I thought ICs were more along the lines of microcontrollers which have things like USB controllers, PWM controllers, GPIO etc. For these, it's mostly programming.
The library I described was supposed to be about all the external bits: signal conditioning, power conditioning, sensors etc.
Well, those aren't the hard bits, you can generally pull them out of a textbook or reference design and expect them to work.
You're overlooking the entire field of analogue ICs. Again, there's a fairly good cookbook of reference circuits out there. In some cases manufacturers will help you a lot: TI have a tool for designing switchmode power supplies around their chips.
So you can compose circuits into schematics. You don't usually compose layouts as it's more compact to hand-redo them for your particular board shape. There is some open source hardware at this level; often to be found on tindie or via hackaday.
Then the other obstacles to shipping a product: regulatory approval. EMC testing. If it plugs into the mains, a whole bunch of safety testing. In the EU, CE marking and WEEE registration. If it has a radio, radio approval.
Economy of scale matters as well. That's why things like OpenWRT are so popular: repurpose hardware that's already mass-manufactured for a slightly different purpose.
