The thing is, most Roblox players that I've seen have FB Messenger (the kids version) video/audio calls running in the background so they can talk. With the new focus on metaverse the combo really makes sense.
On the other hand Roblox could really pull a coup by replicating this functionality natively.
I assume they don't do it because they are scared of being responsible, not because it's technically difficult. It probably helps that the communication is separate from Roblox because it's harder to manipulate a kid on two separate channels together (in the same ways it's hard to actually get two kids connected that want to be connected).
Though Facebook also has some advantage because they are already maintaining a bunch of relationship information and they use that to bootstrap Messenger For Kids since one of the common patterns is parents become Facebook friends with clear identities and then they connect their children.
On the other hand Roblox could really pull a coup by replicating this functionality natively.