Because CJ is a separate seat. It is not based on seniority or merit. It's just a different seat from the others. When Roberts is gone, that seat will open up and a new CJ will be nominated.
However, Thomas is now the most senior member. So, in any decision where he is in the majority and Roberts is not, Thomas has the authority to write the decision himself or delegate it to someone else.
Why this guy isn't Chief Justice I'll never know.
Because CJ is a separate seat. It is not based on seniority or merit. It's just a different seat from the others. When Roberts is gone, that seat will open up and a new CJ will be nominated.
However, Thomas is now the most senior member. So, in any decision where he is in the majority and Roberts is not, Thomas has the authority to write the decision himself or delegate it to someone else.
In Dobbs, he delegated it to Alito.