Ask HN: Why do purchased B2B email lists still have such poor deliverability?

I’ve run into this multiple times and I’m honestly still confused.
I’ve tested purchased B2B email lists (including ones sold as “verified”), and deliverability is often terrible — bounces, low engagement, the usual.
Is the core problem outdated data, weak verification, catch-all domains, or something else?
For people who’ve actually solved this, what approaches worked in practice?