Are you able to articulate 5
or more of those plentiful reasons?
At times I have zoomed in or out because:
1. I'm further away from the screen, because my screen is large.
2. I'm further away from the screen, because I am showing a website to a friend or colleague.
3. I'm displaying the site on a projector, and people at the back of the room say they can't see.
4. I'm viewing a website on my colleague's high-dpi display, which she uses at native resolution.
5. I'm viewing a website on my phone while being driven somewhere, and the ride's a bit bumpy so I don't want the text too small.
6. I have a comparatively low-end phone, that most people don't seem to test for; sometimes the live suggestion dropdown for a text box is obscured by my on-screen keyboard.
7. I have a comparatively high-end phone, and I'm not moving around, and I'd like to have more of the article on my screen.
8. I have a comparatively high-end phone, with a high-dpi display, and the page I'm reading just seems to have rendered everything quite small.
9. The website designer has opted for a skinny font I'm having trouble reading.
10. The website designer has decided a feature is unimportant and should be visually unimportant (maybe a link to some terms and conditions, or an EULA or something like that) but I want to be the judge of that.
11. My eyes are tired.
12. My eyes are fine, but I've had a few drinks and things aren't quite as crisp as they usually are.
TLDR: if a user in a test group says "I'm having trouble reading that text" would you say "No you're not"? Of course you wouldn't, it would be ridiculous to do that, the user has first-hand perfectly accurate information you don't have access to. And yet, when a user expresses that they're having trouble reading the text by zooming, and you disable zooming, that's exactly what you're doing.