Why must I update my operating system to upgrade iTunes?

I am the happy owner of a new iPhone 8, but here's the thing: when I tried to attach it to my computer, I got a message that I needed to update iTunes to version 12.7 or later, would I like to download a newer version?

Sure, fine, but when I clicked on the option to download it, it took me to the "upgrade to OS High Sierra" page. Grr. Then when I got a message that I could not upgrade to High Sierra.

So, question 1: Why can't I download iTunes 12.7 or later without upgrading to High Sierra?

Question 2: I then compared the requirements for High Sierra to my computer, and can't figure out why I can't upgrade. My computer meets all of the requirements, as far as I can tell. What... the... heck?

Any ideas?

P.S. Upgrading your phone shouldn't mean you have to buy a new computer, not unless Apple wants to offer a helluva package.