Phone call forwarding works when all the devices are nearby, and they can handoff the audio streams between themselves; but you can't pick up a phone call on your Mac if your phone is on the other side of the world/at the office/etc; the feature we're talking about here would not have this limitation.

SMS syncing is true. But if you have a phone and you're out and about you also need mobile data, so would otherwise need a second mobile plan. Or to swap the SIM over, which was easier with a physical SIM, more difficult with eSIMs.