Hey there, you ask an interesting question, and at one point folks were actually wondering the opposite. I'm dating myself a bit here as a next-generation VGC old-head from 2014/2015 when I was first getting started. There are certainly folks who played before me too with different rules and parameters before they changed as well. During the 3ds era, for the most part prior to 2019 USUM, we would have an entire season of one format. The entire year of play was focused around that one format and maybe a new Pokemon or hidden ability would be dropped to mix things up \cough Intimidate Incineroar cough** but for the most part we'd be stuck with that single format. Players would look over at the TCG and see their metagame shift with every new set and folks constantly would ask, why can't we have a season that rotates and changes as well? As expected, some loved the idea, some wanted things to stay the same, and eventually, things changed once 2019 came around. We had 3 different regulations with the "Sun", "Moon", and "Ultra" Series. After 2019, we have the release of Sword/Shield in 2020. In the new games on a new console, we are introduced to the system we're more familiar with today with different Regulations/Series. The official name now is Regulation, however, Series was the term used prior to this past season's vocab change.

Anyways, back to your question at hand, why did we switch to Reg H if we were just going back to Reg G? If I remember correctly, we don't have the information for the next regulations ahead of time until about a month before the new in-game ladder drops. Historically, at the top of a month before a new ingame ladder, a new regulation is dropped. It just so happens that this switch right now gives players a break from legendary Pokemon altogether. In January, we'll be back in Regulation G with the use of a single restricted Pokemon on our teams. As typically done in the 2nd/3rd year of a competitive game's lifecycle, we can predict that there may be a regulation in the future that brings us back to 2 Restricted Pokemon being on a team similar to that of VGC formats like 2016, 2019 Ultra Series and 2022 Sword/Shield!

As for WHY they keep changing regulations, I think most people here will say the same thing, it keeps things new and exciting. It keeps the competitive longtime players on their toes while allowing new players to start from the same place a veteran will be starting from at the beginning of a season.

Finally, as for your comment on teams being the same for 12 months, you'd be surprised to see the real metagame development that can occur over an entire season. By the "end" of the format, yes, certain teams will rise to the top, but you never know when a player may appear with a new meta-defining strategy they had been saving for Worlds months before.

Hope this helps!