Idioms are common in every language I know (about 4-5 depending on which you count that I speak reasonably, and a few more I can read with some effort).
> Could the number of "friendly noises" and idioms in English be because most english speakers only speak english?
I don't know. My experience is that a lot of languages tends to have a much more formal hierarchy of terms to use for greetings, while English is all over the place. I think maybe part of the challenge is that English is used by people spread out much more and with very varying degree of experience with the language, including people using it as a second language, and so it's in some respect more internally diverse. It also lacks a strong effort to direct the language.
E.g. I'm Norwegian, and Norwegian has an official organ that gives government and the public advice on the evolution of the language, and language reforms have regularly passed as acts of parliament. French has Académie Française. For a lot of languages there are these official or semi-official organisations that sets out how the language will be taught at schools and what the official use should be like. It doesn't stop proliferation of variants (Norwegian authorities have more than once had to admit defeat and undo language reforms that failed to catch on, or just adjust the language to reflect use rather than vice versa), but it does tend to tame it somewhat.
When I first started learning French a quarter of a century ago, one of my classes was "mercantile French" which included lessons on French business correspondence, and one of the things that stuck with me was how much more formal it was (at least back then) in terms of greetings and signoffs and formatting than English correspondence. I'm taking a refresher course for my French these days, and the perceived more formalistic determination of level of formality is quite interesting.
> you've also the interesting thing where lots of americanisms have been adopted here
Yeah, in this case it may well be part of it.