Someone just uses my sister's 12-year-old account as their own now. Any ideas why they would do this?

My sister and I used to play Roblox a lot as kids, but we haven't played in ages. Today we looked up our old accounts only to discover that my sister's account was recently taken over by someone who has been actively using the account as his own. The username was changed and the account just looks like a normal, active profile.

The weird part is: there really wasn't anything valuable on the account. There wasn't any valuable items or highly-populated games or anything. I don't think my sister ever even had a subscription. This person just picked a random old account, took it over and started using it as his own regularly.

Just out of curiosity: is this a common thing? Does anybody have any idea why he would do this instead of just making a free account?

EDIT: Xysionc_k is the new username for anyone interested