Given there are about a billion electronic P/N's accumulated to date and DigiKey has 3 million of which, there are 10,000 opto-isolators and you are wondering why your free tools don't capture all these? Big companies pay for tools or time to create the common library parts. When I was Operations Mgr for an R&D company, for documentation side, I looked after Eng release controls, Inventory, Standard Database of internal PN's, BOM integrity, stock picks but I told the Engineers to create there own library parts. So they did which included schematic/library/PN creation of a standard part with multiple sources.
With regards to your example there are a couple opto Isolators which easily meet your requirements and in stock. However due to package restrictions there was only 2 out of 10K that matched.
When you build up your design skills , criteria for cost, stable supply/std PN, may change your filter requirements and help minimize your library needs and adding a new part will become a trivial task. That being said, all the basic passive RC SMT library parts ought to be available in generic form. We used internal PN's for BOM so purchasing has flexibility but Eng must specify prefered PN"s and Mfr's.
But I sympathize with your challenges.
added Others in this forum have attemped to define the optimal CAD package, but it all depends on your budget and needs, so it does not exist. Consider that CAD packages range from free to $0.2M per seat, which does mor ethan you specified, your needs may be satisfied with a $5K budget such as Proteus Pro or maybe you just want the Starter kit with 10K schematics, 1500 footprints . DipTrace is one I recommend with 100K library parts, but if you get the free version I know one who created their own who might share if you ask him. Also beware that IPC standard library footprints are essential and not free. Footprints vary between SMD reflow and wavesolder for the same device and designed for most equipment to give fewest solder defects.