How hard is it to make people understand what a business email should or shouldn't include? If you're being asked for data by someone you don't know, either ask a manager or someone connected to the account in question. Are people really so gullible & trusting?
Skepticism and rational thinking require effort, and thus as people are rushed or tired, those are the first defenses to fail. You can train to recognize patterns of phishing, but learning those patterns takes time, repetition and effort.
All that a good phish requires is finding the right buttons to push on the right person, and they have many potential victims to press them on, and little to no consequence to getting it wrong.
So, the answer is people are not particularly gullible, but the weak links are a constantly changing, largely unknown dynamic and the phishers can hammer at all of them simultaneously until they get through.