Because their browser engine sucks, people just use it on mobile (iOS) because they basically have to in order to open any link.

Every full time job I've had I've been issued a MacBook and step 1 of setup was "install Firefox or Chrome".

Safari is one of the few things that truly outright blows about the Apple ecosystem.

Safari is the first thing I launch when I open my Mac and it's the last thing that I quit before shutting it down. If you're looking for battery life and RAM to spare for other applications, use Safari.

Safari is a fantastic browser, actually.

It's as fast as Chrome and Firefox. (Firefox is curiously sluggish on the Mac, even when the latest Quantum improvements.)

Sure, it tends to lag behind when it comes to adopting certain newfangled web technologies, but it supports everything that's important (even web workers now? I think?), and it's super stable.

I use Chrome for web development, though, because of the built-in dev tools. (The tools are pretty gnarly for 2019, but they get the job done, for the most part.)

Because safari will sooner or later become another Edge. Apple doesn't invest enough into safari and now it's a browser which is most behind in terms of technologies.

Apple's Safari investment is focused on features like tracking prevention and anti-fingerprinting. Stuff for users at the expense of developers.

I've seen what happens to web browsers that trail behind the industry in features.

It doesn't matter how nice the privacy features are, when a user encounters web site after web site that doesn't work right in their browser, they eventually each out to a more technical friend that recommends trying browser X. When browser X fixes their problems, they never return.

iPad / iPhone are captive rich markets, where only Safari is available. So companies/devs will keep on testing/supporting for it.