There are a ton of two-sided marketplaces out there that try to do this. It's a messy challenge involving rights management (no good photographer will agree to the contracts these companies offer), production workflows (all companies do it differently) and assignment complexities (by 'high-quality' you mean 'high-quality and custom to the project' because a nice but generic picture is mostly useless).

But you're right in that free photos like the ones from Unsplash are super expensive in the long run because of the search ranking penalty (you get grouped with all the spammers who are using those same free photos).