If I were to make some headcanon about this (other than "magic" like so much else about vampires), I'd say they're "just slightly out of phase" which is why they can't cast reflections...just as they don't give reflections, they can't give the proper air (so it would be like "dead air"). It's a stretch. (And to be more consistent, they shouldn't cast shadows either.)
It might also somehow be related strangely to not being able to enter a human's home without an invite. Can't breathe in without a formal invite to give you CPR (and kissing doesn't count as an invite--actually, thinking of kissing vampires is both gross and hilarious to me, and if for some reason I DID make out with a vampire, I'd be so tempted to eat a lot of garlic just to see if it has an effect, in addition to numbing the taste of dried blood, tiny bits of raw flesh from skin that got ripped off, and whatever else in my own mouth from kissing a vampire).
The simplest could be that since giving CPR isn't something vampires did, it's possible Angel didn't know if he might accidentally turn Buffy into a vampire. This is also a stretch as if their making out didn't do that...but vampire rules in the Buffyverse are oddly very specific in ways that have to be put into the magic category rather than scientific.
EDITED: if I were asked how to explain that scene, I'd explain that away that only a vampire that has recently drank fresh blood could give CPR since they can't generate their own blood (with blood necessary to process the air). They need the blood to smoke, get drunk, and also breathe into the mouth of another. So Angel could say "I haven't had fresh blood recently enough." (He'd be self-conscious enough about it that I could see him dancing around that to say what he did to Xander. )