Because not all of your "fellow developers" have the same priorities as you.
For a lot of developers, targeting just the Apple platforms is still a worthwhile investment in itself: Apple's customers tend to pay higher prices for quality Mac software — yes, outside the Mac App Store, too — and they will very often be happy to pay for the accompanying iOS app if it's worth doing so.
There have been Apple-only development houses since the year dot and this won't change just because Apple decided to release a platform to make it a little easier for its developers; they're not beholden to the rest of the world and it's ridiculous to believe so.
There sure are a lot of developers only interested in developing for Apple platforms. But wouldn't it be beneficial for Apple to extend the framework outside their ecosystem? I doubt the reason they haven't is to limit expending resources.
The direct comparison one might make is to Microsoft, who recently opened up .NET to have official Linux and macOS compatibility. The result is that enterprise and small businesses can deploy Microsoft technology on their Linux servers, possibly moving towards deploying on Azure. The cloud is Microsoft's big money maker — providing cross-platform tooling is one way to support that business.
Apple makes their money from selling devices, not from offering a cloud infrastructure. Therefore, it makes no sense for Apple to offer their high-quality tooling and frameworks for other operating systems — the aim is to have developers selling their software for Apple platforms, eventually having people pay for services like iCloud storage, Apple Music, Apple TV — and then stay with using Apple's hardware. Maybe a person tries an iPad. Then they get an iPhone. Then they get a Mac.
Apple recognised this about two decades ago when they officially cancelled development on the Windows version of Yellow Box (later to be named Cocoa, the base frameworks for Mac and the ancestor to UIKit, the base frameworks for iOS). It has worked well for them.
Is it right or wrong? That depends on one's perspective about lock-in. However, I don't see a problem with it in the sense that nobody is being duped — unlike Microsoft in the 90s practically forcing Internet Explorer on practically every PC available, Apple doesn't have a market monopoly.
Apple's strategy has remained consistent since the early days of OS X: you don't trap users, you make simply make sure they never want to leave. That's the pitch they make to developers, too.
Perhaps they like the things that they're paying for, and no one's tricking them into anything, and maybe they even looked at those alternatives and still decided that they liked what they had. Crazy, I know. Something to consider though.
Some other fellow developers are happy to target just Windows, just PS4, just XBox, just Wii, just Switch, just Android, just Arm mbed, just Tizen, just whatever OS.
Not everyone has this urge to create FOSS software, available as free beer in every possible OS out there.
I don't understand how my fellow developers could ever tolerate Apple doing this.
This goes one way, and its been like this for decades.