Ask yourself, what kind of person chooses to work there?
I frequently do Gordon, pretty much daily

For the ones answering the phones, why are they still doing that if they’re any good?
I'd take the view that Revenue, no different to any other large bureaucratic / hierarchical employer in the private or public sector is a microcosm of the workforce as a whole; in a lot of cases people find their level, some bide their time and avail of family friendly working arrangements, some hugely capable people lack ambition to progress. So, all sorts of reasons - I think it's preposterous to suggest / assume that anyone answering the phones, in Revenue or anywhere else, aren't "any good". It kinda smacks of snobbery to be honest.
Due to the demographics of Revenue, they are recruiting substantial numbers of new staff on an ongoing basis - over 150 new clerical officers (phone answerers in your parlance) in the first quarter of this year alone - this was why I was interested to see if the "zealots" assertion was a longstanding view or something emergent since, to use an Only Fools & Horses analogy, Trigger's sweeping brush has had much of both the handle and the head replaced in the last 5 - 10 years...
They must do a great job of indoctrinating / brainwashing all of the newbies, don't you think, credit where credit's due etc...?!
I’ve met many over the years and maybe 1 in 10 has been in any way impressive; they recruited some good people who were at Director level in the Big 4 post the Global Financial Crisis after they were let go and that helped somewhat but even those people were beaten up to a degree by the bureaucracy and inefficiency of the place.
Anyone coming from Director level in Big 4 would be joining Revenue at Assistant Principal or more likely Principal Officer level, so you are talking about a very small number of people there.
At that level there is absolutely no question of this happening:
e.g. “torblednam, why are you processing so many files per day? You’re making your colleagues look bad.”
No different than any other large organisation, people are generally quite ambitious and competitive at that level. This comment actually demonstrates your lack of understanding of the realities for people operating in the most senior grades.
Interestingly, Revenue has hired a good number of staff, at the junior / mid management levels with Big 4, Top 20 and small / mid-tier practice experience and accounting and/or tax qualifications that they garnered there. These must make up at least some of the 90% of spacewasters that you have encountered... so it makes for an interesting corollary of your assertion - since these make up the cohort of Revenue's compliance staff - what proportion of the people gaining accounting / tax exams in the private sector aren't any good...?
But the biggest issue that I’ve seen over the years is that fundamentalist tendency; they mistake their job for collecting as much tax as possible when it’s is actually to collect precisely the correct amount of tax.
This is very interesting, since there are very accessible complaint and review procedures for people aggrieved by their dealings with Revenue. Have you availed of these in relation to any / many of the zealots down through the years?