One factor might be that web pages are big enough that they begin to allow contributions from different, orthogonal origins. The initial designer might do the

One factor might be that web pages are big enough that they begin to allow contributions from different, orthogonal origins. The initial designer might do the first main layout and bolt in some menus and the main text area. But other people then gradually want to add these extra panes, those hot videos, that extra image viewing layer, this advertisement, all kinds of scripting, new functionality that all comes together orthogonally but will together take megabytes of space. These can be added without going back to reworking the original design too much so people can go in, think "I don't know how the whole page is actually built but I can just add this little thing here and not mess with anything else", and copypaste one more of the latest features onto the page. Or this mechanism could even be automated: new people just add extra page modules to a database and the original workhorse will go rebuilding the HTML with the new additions in it and nobody is left to oversee what all they're actually serving out on each page load.