at the end of 2023 Hamilton would be almost 39. Makes sense to me to not commit for too long for now. Could be that he wants to retire after 2022 or that Max becomes available over exit clauses etc.
might be 2+1 for example.
It could be Mercedes not wanting to commit to an aging driver before knowing if he's really still that competitive by then. It could be Lewis not wanting to commit for much longer beyond the regulation changes (in case Mercedes messes up for 2021 or beyond), or him already predicting he might want to retire. Could be part of a planned transition to focusing on Russell or someone else. Could be exit clauses complicating the deals. Etc.
Still, I would've thought they'd want to sync contracts. This could eventually end up placing them in a very awkward position for the 2023 season if Lewis retires at the end of 2022, but then they can't get Max neither Charles (both signed up for longer already as of now), and Russell/Ocon turn out not to be that great.
But yeah we just don't know a whole lot about these deals and I'm essentially speculating on speculation from the media, as the Lewis renewal deal is not announced.