I suspect that most resorts sell fixed weeks, it's just that most of the big-name hospitality companies that entered the timeshare business in the last 15 years sell floating weeks and points, as part of their "program." Most of these are developers who operate several resorts, and are catering to a different demographic than the original timeshares.
If you look at most of the older resorts (lower numbered resorts in RCI, for instance) you will find that most sell fixed weeks. You will probably also see a larger percentage of owners using their weeks at these resorts, as that's how they were sold - exchanges were and added benefit. Now exchanging is a big part of the hard sell; while a salesman will tout the benefits of your home resort, it will be more in terms of how it translates to demand within the exchange system, not the benefits of actually staying at your home resort.