On desktop there's emails, chat apps, etc. On mobile the default browser is often used for the webview within apps, e.g. on Android I have Firefox doing the rendering for webpages inside other apps' UI, which means I can block ads thanks the uBlock Origin.

It happens all the time, for lots of people.

Mobile is different. But on a desktop, most people use webmails, and many chat apps are web based. I barely click any link outside of a browser on a desktop.

I click on links all the time from email (not everyone uses webmail), console, IDE, etc. Enough that yes, it very much matters. Even if not that many links happen, there's 0% chance you wouldn't end up by default using whatever browser it is that opens when you click one.

Every time I click a link in email, IRC, Slack, Discord... At least 100+ times a day, easily.

You're right, it doesn't matter, they spend so much time and take so many risks (and pay so much in fines) to push their own settings for absolutely no reason.

You know that handy dandy search bar that Windows has decided will occupy your taskbar?