Who? When? Where? I refuse to believe this assertion. Are we talking past or present? The whole reason why these frontend focused frameworks exist is because either the existing solutions (pure html, even with preprocessors like handlebars, pure vanilla JS, lower level libs like jQuery) did not scale to meet demand or existing frameworks (JSP, PHP, etc) became a mess at scale.
Everyone loves to throw out these imaginary success stories of corps rolling just with html/css but I've never seen it actually proven true with evidence.
Your assertion that older frameworks 'became a mess at scale' is a sweeping generalization. Any technology can become a mess if not properly managed; that's not a failing of the technology but of the architecture and development practices. PHP, for instance, powers Wikipedia, one of the top-ten most visited websites in the world, and it handles this scale quite well.
The emergence of frontend frameworks was not solely because 'existing solutions did not scale'; it was also driven by the industry's evolving needs for richer user experiences and faster development cycles. These frameworks solve specific problems but come with their own trade-offs, including complexity and steep learning curves. To dismiss alternative approaches as unscalable without evidence is not just intellectually lazy; it's a disservice to the field of software engineering, which thrives on diverse solutions to complex problems.