Your phone/plan carrier using voice data to make a marketing profile is well documented actually. This data is purchased and verified and resold by meta, or in some cases bought and used by alphabet for GAS. Cacti can show you outgoing data for every device on a network, and you can see data being sent from a phone in signed packets going to your carrier when you're not "actively using" it. It seems like you know about network monitoring tools but you haven't actually used them, just talked about them in reference to data collection.

"Why buy the cow" here is also easily answered: not everyone uses Facebook, a fair number of users will deactivate their facebook page but continue to use messenger.