Why Can't You Just E-Mail Me?
HM Revenue and Customs staff would love to email you, for all the reasons you can list: it eliminates "telephone tag" - it's particularly good for getting hold of punters who don't work in an office. It's quick, it provides both sides of the conversation with an audit trail and a written record of what's been said. If someone has to research an answer, they can go off and do it without keeping you hanging on the phone (or worse, guessing).
Why do they ban email? There are many reasons. The fear of fraud; They don't want someone spoofing or hi-jacking an HMRC email address to fleece you. They don't want you getting so used to dealing with HMRC by email that you let down your guard when some con merchant comes along.
Some people think they just don't trust the staff. They've spent years getting rid of case ownership, dumbing the work down so it can be automated, removing any human intelligence in the construction of letters. They daren't let them loose on emails. Dear, oh, dear. What a suspicious world we live in. Perish the thought.
But, I hope you get the point. It may be annoying for you, if someone from HMRC says they're not allowed to email you. It's twice as annoying for them having to say it .