I took a look on the GE website, and apparently for Excellence Class they release seats more than 93 days in advance. Here is what it says on the website:
Reservations for Glacier Express in 1st and 2nd class can be made at the earliest 93 days in advance. The Excellence Class on Glacier Express can be booked online for the entire timetable period.
https://www.glacierexpress.ch/en/menu/services/faq/#:~:text=Reservations%20for%20Glacier%20Express%20in,for%20the%20entire%20timetable%20period.
Most likely the tour companies book the seats as soon as they are released, and then later release the ones they have not sold to their clients. This reflects the highly tourist-promoted aspect of the Glacier Express and “Excellence Class” in particular. Tour companies can grab all the expensive seats, and then use this to promote their tour. They can make a lot of money on this.
But do you really want Excellence Class, or the whole Glacier Express in general? I have been on a platform at Betten, where the GE does not stop, watching the GE pass slowly by while we waited for our regional train. Honestly, everyone I saw through the windows on the train looked bored. Some were asleep. I could see this from the platform because the train is on either rack rails at this point, and going very slowly.
There are lots of great experiences to be found in Switzerland; do you really want this one? The regional trains travel the same tracks with the same views, but without the pricy extras.
Edit: Regarding the comment that “tour groups, probably like the ones posters on here use, are the reason it is so difficult”, I will note that the Rick Steves tours do not include the Glacier Express Excellence Class. Or maybe not the Glacier Express at all; I have no idea as we have never taken a Rick Steves tour. Actually, many or most of the people who post here, including myself, are independent travelers and do not take guided tours of Switzerland at all.
The tour companies that do book the Glacier Express Excellence Class are most likely the ones that promote luxury travel with fancier hotels and concierge services.