The reason not to use underscores is because people don't understand them. I learned this the hard way back in the late nineties the first time I set up an email account for a business. I chose yyy_architects@emailhost.com instead of using a dash because of the connotations the underscore carried regarding computer programming conventions. Then I enjoyed the opportunity to explain over the phone exactly which key was required to correctly type our email address many many times. [Stylized conversation]
It's an underscore not a dash
Huh?
It's the the same key as the dash, only hold shift.
Huh?
See the dash key?
Yeh.
Just hold shift when you press it.
Huh?
Do you have our fax number?
That's why you don't use underscores in a URL. Of course, today nobody uses dashes either if they can help it.
If you're optimizing for search, the dash-separated words are very common. Google likes you even better if you make the most important thing first in the URL. E.g.:
I remember seeing news broadcasters in the 90s that didn't know how to pronounce the '@' symbol. So while I find your story all too plausible, I have a hard time believing that people still don't know how to type an '_'.
I suggest that there are fewer people in this world that 1) know how to use a search engine, and 2) are incapable of examining their nice silkscreen printed keyboards, than you suspect. There is a world of difference between "uses it" and "capable of figuring out how to use it".
