Are they? Have web browsers actually gotten any faster at all over the last, say, 5 years? 10?
JS engines got a bit faster, but what about CSS & HTML parsing? 2D rendering performance? Layout engine performance? DOM performance? Mozilla made a bit of noise about this a year or two ago with their whole Project Quantum push - but had you ever heard a peep about this stuff prior to that? Or since? Nobody benchmarks this stuff, and yet it's insanely critical to interactive performance. But since it's harder to measure than JS performance, the only thing ever measured is JS performance. And occasionally, rarely, page load speeds.
Open up a 10MB plain text file in Chrome and it completely falls over. Zero JS. Zero CSS. Zero HTML. Just plain text. Are modern browsers really fast?
And for what it's worth modern computers are wide - 4 core with SMT is damn near low end these days. Yet the web is still incredibly stuck in the single-thread mode of operation. Both the browser internally and the platform itself (WebWorkers are far too slow, heavy, and restricted to meaningfully be used to offload interactive work). And there's almost no work being done to address this. WASM's threads are the only sliver of light here on the platform side. Is it really surprising that people throw RAM at the problem as a result? Throwing more caches at things is the natural response to being heavily starved for CPU on the single thread you can use.