PO - "We need an alert dialog, just some text." Dev - "Cool, gimme a few seconds... and there you go, window.alert()" PO - "No it needs to match our styling and look exactly the same no matter where it's running." Dev — "sigh."
Meanwhile, a bunch of the most successful sites are ugly as sin. But whatever. I'm sure it's worth thousands of extra dollars (though not enough thousands to also match all the built-in functionality of the feature you're replacing!) to make the datepicker pretty, but break a11y and keyboard shortcuts.
The base platform is pretty ugly and feature-starved, though. Like, date/time formatting and conversion to the user's preferred TZ should be a built in to HTML rather than re-invented a thousand times in JS or server-side. Some basic table sorting feature that'd serve 90% of all needs ought to be built in. Maybe alerts could allow more styling while still being accessible and safe. The datepicker widget should be good enough that no-one feels the need to make their own—though to be fair I've still seen that crap happen on mobile, where the platform widgets are basically fine, though part of that was Android's fault for sometimes having god-awful datepickers depending on version and vendor. The default styling mostly is bad (alerts actually aren't too shabby these days, at least in some browsers, though). It's a crappy state of affairs.