

GDPR and pii reasons most likely. It’s a nightmare keeping track of why certain data is on certain accounts. This can vastly simplify the GDPR compliance mechanisms. If your GOG account is merged with your PR account, there is probably significantly more “sensitive” data (CC numbers, addresses, etc) in the GOG account. This probably exempts some data that either cdpr or gog tracks from deletion or retrieval requests.
Separation of data between accounts makes them fall under different retrieval requirements.
As one account, a request for all of the data from that account contains both chunks. Separation of those accounts separates the need to accommodate requests for data from one on the other.
It can also mean that internally they may have a sufficient mechanism that data that was previously identifying to no longer being identifying (breaking userid to data pairings for example) which is sufficient to “anonymize” the data that it no longer needs to be reported or maintained.