Because the [Windows] market is driven by price (or cost). There's no appetite in a large enough volume to produce a "high quality" [Windows] laptop. Or at least that is what OEMs believe, likely based on some form of market research.
Today, we all know that wasn't the case, as (often pricy) mechanical keyboards are (thankfully) back in vogue.
It was just lack of availability the whole time.
(Yes, my purchasing decisions are very keyboard-driven.)
They're great, but still a niche. And extremely rare on laptops.
Perhaps a niche, but it is strange how objectively bad keyboards got before literally anyone complained, even though in retrospect we all look back at those terrible keyboards with disgust.