Well, yes, but being able to designate a VPN node as a Tailscale exit node directly means you don't need a random server in the middle for it. (Which is beneficial if you use it as an exit node for road warrior devices)
You can, but the issue is usability..when I'm watching TV, I want to just be able to flip open an app and say "I'm in London!" and watch BBC, then the same for Canada and etc. I don't want to be fiddling with a VPN and switching routes on some separate device / switching the entire wifi network or etc.