I mean it's all the same account and servers, ffs. It's like they went out of their way to complicate things.
I mean... it's clearly not?
The most simplistic way to look at it is that they might have different databases (or database tables at least) for the progress data. Maybe there's differences about how the 3 database is set up because it matters for 3, that they don't want to touch in the 2 database because it would break 2. Maybe they track different things about your progress. Maybe they've made each record take up less space.
(EDIT: It could be as simple as something so obvious I didn't think about it:
having to track your progress in 3.)
For whatever reason, it's different. They'd be transferring your progress from the 2 database to the 3 database for this process. Something like that.
And they
could do that transfer automatically, but it would mean:
1. Wasting a lot of time running this for every single person who owns Hitman 2, when not everyone is going to be buying 3 at launch and so doesn't actually need the transfer.
2. Arbitrarily locking everyone's progress in 2 to be transferred at this specific date, when not everyone is going to be buying 3 at launch and may play more of 2, then want to transfer that additional progress when they buy 3.
It is pretty easy to see why they'd have to do something like this. It certainly could have been handled better while still doing it this way, but this doesn't run on magic, there's technical reasons to do things like this overall.