As mentioned before, they are quite rare, and it might be more rare to find one that lines up with a week and room type that you'd like at a price point that's acceptable. Looking at some of the numbers just from Riviera and the new VGF2/Big Pine Key from this monthly report:
https://www.dvcnews.com/dvc-program...n-club-direct-sales-edge-upward-in-march-2023
So far at Riviera, only 111 fixed week contracts have been sold.
So far at VGF2 / Big Pine Key, only 39 fixed week contracts have been sold.
I have no idea how many have been sold at other resorts.
I bought 2 fixed weeks at Riviera. There are certain times that I'd like to have a WDW vacation, so a fixed week generally works OK for me. Also, I may overthink things, but long term, it might become more difficult to book the dates you want at Riv. As more contracts become resales over the years, you'll be competing with more people who can only book at Riv.