You're assuming that any OBC also equates to an equal amount of extra pricing, which even it were true it would be difficult to know, unless they kept a meticulous spreadsheet of all the cruises in which they were interested, which few passengers do.
So in most cases people look at the quoted price, and if it comes with OBC then they are happy that they have got something extra, although I do agree that a lower price would be my preferred option.