Why isnt backwards compatibility a standard feature?
Backwards compatibility tends to be a feature console manufacturers advertise at the start of the console's lifespan. A good example being the original Xbox backwards compatibility with the Xbox 360. This happened really early on in its lifespan and at some point Microsoft just stopped doing it.
The reason it's not a "standard feature" is because there's very little money in doing it, and it costs a lot to implement. People generally speaking don't care about backwards compatibility after a console's library has a lot of titles. It's also something that rewards the used video game market which is something console manufacturers have worked tooth and nail to try and kill. Backwards compatibility is also not a trivial thing to implement. A video game console isn't a VHS player, it's technically a computer with a CPU and an operating system. The Xbox 360 ran on a PowerPC CPU wheras the Xbox One uses an AMD x86 CPU. These are completely different CPU architectures and it's not possible to run code on both unless you recompile the game to run on it. This is often in some circumstances completely impossible to do since modern gamedevs don't generally keep source code in a compilable state unless they plan to re-release the game later. Which is only starting to happen more and more frequently.
The only way to do backwards compatibility in this circumstance is to either include hardware from the original console in the box (which increases its size/cost of manufacturing. The PS3 was a perfect example of this where they tried to sell it for 600$ but it actually cost 900$ to manufacture at the time). Or they use software based emulation. Which tends to be spotty/imperfect and is less ideal. You also generally need to do this for each individual game. Which is how the Xbox 360 and later the Xbox One did it.
When will the madness stop?
With the rise in digital companies have found it's far more profitable to just sell the game again on a newer platform. And with the reduction in buying games at retail (We will eventually see Gamestop go out of business it's just a question of when) it's really hard to even conceive of backwards compatibility still being a thing in the future.
What's ironic is what I used to say to illustrate this argument was "Imagine if VHS didn't beat Betamax and they both sort of hung around and if you wanted to watch the Star Wars trilogy you needed Betamax but if you wanted to watch the Back To The Future trilogy you needed VHS!" but now with competing streaming services its already happening. Free market competition is overrated, we need standardization.
A VHS is a completely different device to a game console. It's essentially a "dumb" device you put a standardized tape with video/audio on and it will play. A game console is essentially a computer that runs code.
Not just that but consumers generally speaking don't respond to "standardized platforms". There was an attempt at doing this called the 3DO. Which was a game console that was designed so there was just 1 open architecture that different manufacturers would make consoles for. This catastrophically failed because there just weren't good exclusive games for it. People only flock to consoles to play exclusives and very little else. The main reason the Nintendo Switch is selling extremely well right now is solely because of 10 or so titles you can't play on any other device.
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Now the reason PC doesn't have this issue is because modern PCs still use x86 processors which computers 20-30 years ago also used. The Windows operating system also prioritizes backwards compatibility with 32bit applications. Not specifically because of gaming though but because a lot of Windows users especially businesses rely on applications made 20 years ago that it would cost thousands of dollars to upgrade off of. It just so happens that video games as a result also get supported even though it's also imperfect there.
There's also examples of PC platforms that completely neglect backwards compatibilty. A good example being MacOS. After Apple switched to Intel CPUs in the mid 2000s, it became impossible to play any Macintosh games from the 90s on any newer Mac computers.