[NSAID]

Not in the US. I've received one or two important letters from the IRS (that is, I unintentionally owed them money) that arrived via regular post. I'm not sure if that speaks to the reliability of the USPS or a lack of imagination on the IRS's part (or maybe the government trusts the government postal service?). Certainly people and businesses do use registered and certified mail, but not everyone.

[hakfoo]

I think it's a matter of "they'll try normal mail, then if it doesn't get a response, they'll resort to registered mail."

It's a legal CYA matter probably, more than a government-collusion thing- registered mail comes with a tight chain of custody ending in a signature confirmation, so you couldn't claim "I never got the notice" when they started to ask for a judgement.