You are taking the point of view of a web developer, not a web user: a web browser gives a web site's developer a lot of control over the experience of a visitor to the web site; the visitor has some control over his or her own experience, but not much. In contrast, an Emacs user has a lot of control over his or her own experience.
Simple example: Emacs is of course used to view files not written by the Emacs user. It is the sole decision of the Emacs user what size, color and thickness/thinness the text will be. In contrast, the writer of a web page decides those things, and maybe the user can modify that and maybe they cannot.
In Chrome for example, there is no simple way to increase the text size without also increasing the size of all the other elements on the page (without increasing the zoomLevel, in other words). If some of those other elements are fixed-position elements (elements that remain in the viewport no matter how much the user scrolls) that already take up a large fraction of the viewport (which happens a lot when you browse the web on a 1360-pixel-wide monitor and you have old eyes and consequently need large text), it might not be possible to increase the zoomLevel.
Simple example: Emacs is of course used to view files not written by the Emacs user. It is the sole decision of the Emacs user what size, color and thickness/thinness the text will be. In contrast, the writer of a web page decides those things, and maybe the user can modify that and maybe they cannot.
In Chrome for example, there is no simple way to increase the text size without also increasing the size of all the other elements on the page (without increasing the zoomLevel, in other words). If some of those other elements are fixed-position elements (elements that remain in the viewport no matter how much the user scrolls) that already take up a large fraction of the viewport (which happens a lot when you browse the web on a 1360-pixel-wide monitor and you have old eyes and consequently need large text), it might not be possible to increase the zoomLevel.