Apple is the biggest example in the tech industry right now of vertical integration; that a company controls not only the end product that they sell but also the component parts.

That's why it's best to think of Apple not as a software company but as a vertically integrated hardware company: Apple provides services (iCloud, iMessage, FaceTime, Siri) to add value to their software (macOS, iOS, watchOS, tvOS) in order to add value to their hardware (Macs, iOS devices, Watch, Apple TV).

To answer the question of why web browsers aren't allowed on Apple TV, it's because Apple does not deem that they add value at any of those levels — the Apple TV is not positioned as a general purpose computer but more as a sort of console, suited to consuming multimedia and playing games but not for general purpose web browsing.

But it isn't as though Apple hasn't anticipated the desire to see web content on the big screen. That's why macOS and iOS devices support AirPlay Mirroring.

I think there are a couple of generations that are unused to the idea of vertical integration in technology and view it with suspicion. Once upon a time, say the 80s and prior, it was the norm: you bought a computer, could only use their OS, and in certain cases could only use their software.

IBM PCs and MS-DOS broke the industry free of that mould in a major way, but Apple has proven to have some success with it — in fact, it's almost the entire value of what they propose: tight integration between hardware and software to provide, as Apple sees it, the best experience.

To come back to the first question: iMessage is both a service and software. It benefits Apple more to use iMessage to sell Macs and iOS devices; there's no benefit to Apple for people to use iMessage on Windows (or Android, Linux, etc.). Apple is, remember, not a services company; it is a hardware company. iMessage "sells" macOS and iOS which, in turn, sell Macs and iOS devices.

In fact, I don't even think Apple sells hardware. Apple sells experiences. That might sound airy-fairy, but it corresponds well to their approach.

Contrast this with iTunes. iTunes exists on Windows in order to sell iPods (and later, iPhones and iPads) and multimedia content to Windows users. This benefits Apple in a way that allowing iMessage on Windows would not.

Same reason why Apple TV+ and Apple Music are available on competing television devices and on Android: selling subscriptions to non-Apple users still provides Apple with a benefit (and valuable opportunity to advertise) that iMessage on other platforms would not.

This approach exists in sharp contrast to both the Microsoft of the past and present, as well as web-based service companies. Their goals are different.

Microsoft made MS-DOS and Windows available on as many types of computers as possible in order to sell Windows licences; Apple tried this once in the 90s and didn't make enough money to justify the operation.

Services like Facebook, that freely provide their messenger services on practically all platforms, have a different measurement for whether or not there is any benefit: they aren't trying to sell you on hardware, they're trying to sell data to advertisers.

There are other examples: LINE, for instance, doesn't necessarily need to sell users' data to make money; instead, LINE operates a content store selling additional functionality.

Therefore, it's impractical to compare Apple's approach with iMessage to that of competing messaging services. The apps and associated services serve different purposes to meet different ends.

To conclude: iMessage is there to make you buy an iPhone.