To who? What evidence is there either way?
Is it an often-repeated story online? That sort of information is both well known and unreliable - it's well-known misinformation.
Big Tech companies operate much more complex systems (for starters, they actually build greenfield stuff instead of buying ready-made equipment from a vendor and plugging it in) and have way less headcount.
You're building new cell towers, managing countless failed backhual links (thanks to fiber's natural enemy, the backhoe), working with whatever obscure bugs your MVNOs have managed to uncover, certifying new cell phone designs, and still working on upgrading everything to 5G while simultaneously planning for 6G (keeping in mind that the 5G network architecture looks radically different than the LTE architecture). Much of that work is necessarily physically distributed across the entire country.
Not to mention dealing with end-user sales and support, which unfortunately often needs physical stores.
I'm not going to say whether 100k is too many, but there's a lot more involved here than just maintenance and monitoring - especially if you want your network capacity to keep up with growing demand.
The problem is that this is a culture problem and once a company is ossified it is really hard to enact such change from the inside even if you wanted to because everyone enjoys the status quo (and who doesn't wouldn't be there to begin with).
Another example: have you seen the UK & EU banking scene and the boom of fintech and "neobanks" around 2017 like Revolut, Monzo, Starling, N26, etc? They managed to build from scratch on relatively shoestring budgets their own implementation of a consumer bank, something that their legacy competitors still can't replicate despite having way more budget and resources.
Unfortunately, the telco world is an oligopoly and they don't like new entrants (banking in the UK was actually a much more level playing field in comparison), so we can never actually see an experiment that proves or disproves my theory.
Yes, this explains most of our jobs.
I also note you kind of glossed over the 'legacy sludge' technologies that telcos must support — that likely takes a lot of manpower, making it very difficult for them to get to the 0.1x staffing level
I can't even begin to imagine what those 100k people actually do. For starters, none of the telcos actually develop their own equipment - they buy pre-made from vendors like Ericsson. Often that includes ongoing maintenance too. The only "engineering" is building the back-office and customer-facing UIs, and even that is often outsourced (as a rule of thumb, if something can be outsourced, telcos will do it: https://berthub.eu/articles/posts/5g-elephant-in-the-room/).
Customer service might be part of that number (assuming that too isn't outsourced), but even then 100k feels extreme.
10k is ok although leaning on the more bloated side. But 100k?