How is it that a organization with these resources cannot manage this?
Is it unwillingness or is it too hard?
They receive 5 billion items of content daily.
The A transparency report for combined FB operations (no, they don't deserve the M-name) is here: https://transparency.fb.com/data/community-standards-enforce...
For 2021Q3, > 770 million spam items were filtered. Given 5 billion submitted items per day I'd suggest that this number is too low a fraction. In a roughly equivalent domain, email, upwards of 90% of all delivery attempts are spam and have been for years.
Which somewhat answers your question: insufficient resources devoted to task.
Spam is down markedly from 2019 (2.5 billion items/quarter), and it's also possible that other actions (account actions) are reducing total spam occurrence. Interactions such as this are difficult to assess.
Note also that FB don't indicate spam "prevalence", the firm's term for "items * views", that is, the number of times persons on the system were presented with spam messages. That would also be a very useful metric to see.