Why are apps like Spotify, Firefox, and Chrome missing in the App Store?

The developers of those apps have chosen not to offer their apps for download in the Mac App Store.

M1 Mac mini, macOS 15.4;  Watch, series 10 GPS-only, watchOS 11.4; iPhone 16, iOS 18.4; TV 4th Gen, tvOS 18.4 HomePods, audioOS 18.4 and Airport Express w/ Airplay 2

Developers don't have a choice when it comes to the iOS/iPad versions of their apps. The iPhone and iPad are closed ecosystems, and the only way to install those apps is through the App Store. But the Mac isn't a closed system, so the developers don't have to use it.

I feel your frustration. The only solution is to run an emulator like QME or VMware then port to a Linux or Windows emulation. From there you can run your favorite browser, but you might not be able to connect to the internet using your wifi without a patch. I'd say the easy way is to safe-boot into an external drive with Linux using "options-key" >external SSD. you can grab Linux-ubuntu pre-installed USB versions from Amazon. Don't try to build boot disk it yourself as Apple's usb-writer doesn't work as well as Rfus-usb writer. If you know how to use all these then great. I'm not keen on apple store as many windows or linux apps don't exist so you have to use work-around. The way I get around it is to use apples RDP remote desktop and connect to another machine.