To elaborate with some context: large sites do this to avoid hammering a small site if posts containing the link go viral.

Like imagine the thumbnail were fetched every time a link appeared in someone's Facebook/Twitter feed. That could be tens of millions of hits easy.

It's also a privacy leak - the target page would get to know about every thumbnail view (a la tracking pixel). Although it's likely they only care about keeping that data so they can sell it for themselves, rather than actual privacy.

I'd rather the target know it than Twitter or Meta TBH.