I can understand why those more insulated from the coalface of email marketing may get confused with the seeming reluctance of some email marketers to increase their email frequency.
Taking a detached view of the numbers, might lead some to suggest that purely increasing the amount of emails you send to your list could increase your revenue.
This can sometimes work, and as long as you are closely monitoring open/click rates and engagement metrics like response recency, you could be OK.
When upping frequency, inbox ‘placement’ also needs close monitoring, as it can lead to less engagement and by default ending up in the junk folder.
The coalface marketer knows this, and it is this knowledge of the possible implications of over-mailing that hold many back.