Non-zooming sites can (and should) honour your system-wide font-size setting, giving you the ability to use the system's settings to achieve uniformly comfortable reading experiences across websites and apps.

Zooming doesn't do much to give users much control over the reading experience -- iOS does not let users set or change the viewport size, so everyone gets the same columns to zoom into/out of.

If you want text bigger than the single size the designer decided on, you're forced to scroll along each line of text -- which is a terrible experience.

If you wanted it smaller, zooming out won't 'reflow' the layout, so you'll just be losing screen real estate to peripheral content.