Sounds like you're working with a discount cruise site or a big box cruise site. But that happens when you're just 'shopping the field for the best deal.' You're better off sticking to the 'cheap cruise websites' for that. When you say you're shopping for the most onboard credit I assume you're just looking for TAs who have MNVVs to sell. That's not how you build a relationship with a TA.
When you work with a really good TA they are there with you from the time you book through the time you return home. And then they start to know you so they can find vacations that might appeal to you on cruise lines and on land. We worked with the same TA for almost 20 years and you build that relationship that pays far more dividends than shopping for the best onboard credit. I know our TA has saved their clients hundreds and thousands of dollars on overall vacations beyond just the cruise and/or onboard credit.
And for those who say you should find a local TA, that's not bad, but with today's digital age, our TA is servicing clients all over the US and even some clients abroad. WhatsApp, Zoom, Google Meet all make face to face meetings so easy with the added benefit of screen sharing. You can find a good TA most anywhere today. 🙂