Some things can't be done on the top level. E.g. anti-tracking and metering. Also because airtime is a scarce resource it's not as simple as "just give me a lo

Some things can't be done on the top level. E.g. anti-tracking and metering. Also because airtime is a scarce resource it's not as simple as "just give me a lower layer and I'll run VOIP". The requirements (and performance and reliability) of voice calls is higher than skype over an IP network on mobile. E.g. there's a reason SCTP is actually used here. Phone networks are in some ways rightly very different from pure packet Internet. Sometimes just for historical reasons from the olden times, but often also justifiably so. I could go on and on, but tl;dr: it's not that simple, but you're also not wrong.