Is it because every character in Strings are UTF-16 and every Number is a float64, the sheer number of objects in the average page, or because JS is a VM language? Something else?
@see
https://tc39.github.io/ecma262/#sec-terms-and-definitions-string-value
https://tc39.github.io/ecma262/#sec-terms-and-definitions-number-value
The shear complexity of fetching resources, parsing them, tokenizing them, node traversing the HTML Nested Div's and to fetch and arrange all HTML tags and their data for building GUI Widgets from its Tags, class's and ID's, CSS Rules, Attributes, Media files and converting JavaScript is all a massive undertaking. Then JavaScript which consist of an entire computer language to set after render and pre-set triggers is just as massive. For over thirty years now HTML Rendering Engines have been transformed into massive libraries all being added to at times daily likewise.