The Roku app is actually really good at determining where everything ever is currently streaming (or purchasable). It's not 100% perfect, but it's generally correct. I go there first for basically everything.

For example, it tells me Wake Up Ron Burgundy (which isn't even a "real" movie) can be purchases on Prime, Fandango, or iTunes.

Actually, iTunes and Prime have mostly everything for rent, what movies were you actually looking for?

Just wanted to watch a relatively recent movie yesterday (Antichrist) and the only place that has it is a streaming service called Mooby. Not signing up for a service to watch one movie. Would have gladly paid 4 bucks to watch it, so had to find it in the usual places instead.

My spouse loves watching old direct to home video movies and I'd say about 75% of them have no accessible copies outside maybe eBay. Most of the remainder are only available via piracy. A vanishingly small percentage are available from streaming platforms.

One that I wanted to watch recently was Disney's 2000 animation short John Henry. It's now part of the American Legends compilation, which is only available for purchase on Amazon, not rent. It's not even on Disney+.

It gets worse when you're looking for a movie only to realize it's different from what you remember. Either a change of substance (different scenes), or a change of form (adjusted color palette). Occasionally the original version is no longer officially available anywhere.

IMDB does something similar (it at least lists Wake Up Ron Burgandy as available for rent on Prime, doesn't mention iTunes or Fandango but I'm not logged in to IMDB so I'm not sure what's enabled/disabled as "preferred services" by default)

Plex does this as well.