The reason you don’t get it is because this studio is doing something that basically won’t please miserly investors or pesky gamers.
Under Zenimax’s leadership, they’ll likely debut a new ES or Fallout every few years. And Fallout 76 has already shown where it ends up just as other studios have shown that messing with engines and releasing games too early is not conducive to public confidence. Certainly not for a studio already known for its gaming mistakes.

What you also see happening left and right is that people who have worked for years, sometimes decades, at a big studio on the most exciting games leave suddenly under the guise of ‘creative differences’ and often start working in smaller studios or even a new studio.

Same for Rockstar for example.

GTA is made by Rockstar North and Red Dead is made by Rockstar San Diego. 2 different divisions produce two very different games.
It doesn’t make sense for a studio with at least 100 experienced people, let alone a second team Todd Howard and team, to produce a successor to one of the best, most successful and influential games in the RPG genre ever made.

With two games in parallel, hopefully you’ll have a thing or two back up.

Not how it works. risk to the investor. In this case Microsoft / X-Box.
If two teams within a studio are busy with two games and one fails, that team may be given the opportunity to start a new game, or a redundancy may follow. Every game is an investment that should make money.

Bethesda has been given a somewhat unique opportunity to put the factory mill with static IPs on a side track for a while and fully focus on two things that have been on top of the heap for years but haven’t been (properly) picked up.
The first is the engine, which was constantly being updated, but could not keep up with some of the latest developments, both technically and in terms of content.
The second depends on the first. Making a game where people can incorporate all the ideas that have been on the shelf for years.

Then they could have been developing ES6 alongside Starfield development for a long time.

They’ve been doing that for a long time.
Bethesda has always had a small team on the side preparing for the next game.
In 2018, they already know where you will play and, perhaps, what will be the main goal. They really started with the concept art and even the ES6 music was already in the trailer.