It's almost certainly a lack of will, or an insistence on filtering in a particular way. (Eg. considering themselves above heuristics, insisting a fancy AI must catch it)
To give you an example, one piece of spam that hit my Gmail inbox recently came from an address like: 6UGH578FDJ3@abusejan.beauty
On that email, Gmail shows a warning that the sender's domain didn't encrypt the message. The email is in a mixture of German & English. Most of the email is one big image. The unsubscribe link text is: "To be removed from our mailing list please?Click Here? Unsub-HERE"