There is another practical reason. It would be a rare situation where Marriott could sell a week for "three or four times" the resale ROFR price.
Most Marriotts sell on the resale market for well over 50% of Marriott's current selling prices. Thus, in the typical low-priced resale ROFR situation, Marriott might be purchasing a week for about 50% of its current selling price. Since that's a little more than Marriott's typical costs for a week that it builds, it's not economically feasible for Marriott to make a market for those weeks, versus concentrating its efforts on new developments.
As for salespeople moving on to another development, that doesn't close out additional sales of weeks at the old resort. Marriott's telesales people in Florida sell weeks for virtually all Marriott resorts, whether new or completely sold out.
Bottom line: Marriott's general policy on exercising ROFRs seems to be related to how much it needs inventory at a particular resort in a particular season.