Because SIMs require the physical card. You cannot forge a SIM without the SIM itself. There's a physical second factor. Usernames and passwords get leaked all the time, and since Reddit et al are meant to be accessed by multiple devices, device-locking them would be antithetical and anti-consumer.
A phone service is implicitly device-locked (well, doesn't have to be, but it doesn't make sense to allow multiple devices per number anyway). Just using device credentials could allow anyone to log in with that information and use your subscription or pose as you.
As for ADSL or other forms of internet, there is already a second factor - ISPs generally know from which vicinity you are connecting from, and when you log in with your account information in your modem, they can match what's on your account to your physical "drop" and assert that it is at least within the same neighborhood - again, creating a physical second factor. This is also why your internet service generally needs to be "moved" whenever you change your residence.
A SIM card allows the freedom as the GP mentioned, without requiring someone from the centralized authority to process the request somehow.
There is a huge difference if you think about it for a bit.
A phone service is implicitly device-locked (well, doesn't have to be, but it doesn't make sense to allow multiple devices per number anyway). Just using device credentials could allow anyone to log in with that information and use your subscription or pose as you.
As for ADSL or other forms of internet, there is already a second factor - ISPs generally know from which vicinity you are connecting from, and when you log in with your account information in your modem, they can match what's on your account to your physical "drop" and assert that it is at least within the same neighborhood - again, creating a physical second factor. This is also why your internet service generally needs to be "moved" whenever you change your residence.
A SIM card allows the freedom as the GP mentioned, without requiring someone from the centralized authority to process the request somehow.
There is a huge difference if you think about it for a bit.