“Mobile-first” has been standard advice for well over a decade, and it still gets skipped often enough to be worth asking why.
Usually it isn’t a deliberate decision — it’s that design review happens on a laptop in a browser on a wide monitor, because that’s where the decision-makers are sitting. The mobile version gets checked last, as a smaller version of the desktop layout rather than its own design problem.
The cost shows up in specific, measurable ways: tap targets too small to hit reliably, forms that are painful to fill in on a phone keyboard, navigation built around hover states that don’t exist on touch screens, and pages that load fine on office wifi and slowly on mobile data.
Designing mobile-first doesn’t make the desktop version matter less — starting from the more constrained screen forces clearer decisions about what actually matters on the page, and that discipline improves the desktop version too.