They do this for a reason which is to protect their customers (they will tell you) as well as to try to convince you to not drop the line. Allowing you to cancel a line by email could cause a ton of issues such as, someone else cancelling your line, them cancelling the incorrect line, them not cancelling it correctly and causing you to incur fees, you sending in a request 1 day too early resulting in ETF. Many businesses like this want the voice interaction as it is recorded so when you complain that they cancelled your line without reason or authorization, they have something to fall back on. eMail is way too easy to spoof and they do not know who they are dealing with.
You used to be able to cancel online a long time ago and they ran into all of these same issues as well as people going to see what the cost to cancel would be and mistakenly cancelling their service or line.