The rationale I've seen elsewhere is that it saves money. It means you don't need to go to the effort of downloading, storing and updating your copy of the database. You can offload all of the externalities onto whatever site you're scraping.

My guess is that the scraping tools are specialized for web, and creating per-application interfaces isn't cost effective (although you could argue that scraping Wikipedia effectively is definitely worth the effort, but given its all text context with a robust taxonomy/hierarchy, it might be non-issue.)

My other thought is that you don't want a link showing you scraped anything... and faking browser traffic might draw less attention.

Money. One requires you to use your hardware and your developers. The other way doesn’t.

It's possible that they don't know. I literally didn't know there was a full downloadable db until right now.

Because that would probably require extra work, why do that if it already crawls and scrapes it in the first place

i tried doing that in summer 2019, and the downloaded formats were at that time proprietary and depended on decoders which were like a tail recursive rabbit hole.

in contrast, letting their servers render the content with their proprietary tools yields the sought data, so scraping might be a pragmatic choice still.

Sheer laziness?