Why is it possible for all programs / apps to see all network traffic?

I am reading about sandboxing, specifically for Android and Linux based systems (like snap apps). Each app is isolated and can only see its own files, i.e. each app has its own environment. What I don’t understand is why can each app see all network traffic being sent? On Android I can install HTTP Canary which works by being a VPN and then allows you to see all traffic sent from your device. On my PC I can use Wireshark and monitor all traffic sent from my computer. My question is, why is this possible? Why do all programs have the ability to see all network traffic? Shouldn’t true sandboxing result in only each app being able to see its own network traffic? I am thinking that it’s because all programs have access to the network adapter, i.e. all programs should be able to use the network adapter, and thus each program can see everything that enters and exits the network adapter. Wouldn’t it be better if some form of channels were used, so each app can only see its own channel in the network adapter? I know that as soon as the traffic leaves the device, every device nearby can monitor the wireless traffic, as it is in the air (it’s encrypted however). However it’s only before it leaves the network adapter that I don’t understand, why all programs can see all traffic.