Why don't laptops have proper low power states
Actually, they do, it's called the "S0" low power state, and it's part of the ACPI standard. Microsoft calls this "Modern Standby" in Windows (and "suspend to idle" in Linux) , and it's pitched to do exactly what you've described.
The only problem is, the implementation sucks. Most users actually hate the S0 state because it consumes so much power - on some laptops, even the fans may continue to run on S0, and your laptop may overheat if you've closed the lid and chucked it in a bag, and it's in the S0 state.
Also, because Microsoft and Intel have been pushing this so much, the "standby" mode now defaults to S0 instead of S3 (which is full suspend-to-RAM). So many users actually actively seek to disable S0 and go back to proper S3 standby, via registry hacks etc.
So why is S0 so bad? Part of this is due to the limitations, long history and the variable nature of the x86 platform. All the power-saving stuff was implemented as an after-thought - both at the hardware and software levels. Whereas ARM, at least the modern ARM ecosystem, was developed with mobile usage and power saving from ground up. An x86 PC is also made up of components from disjointed manufacturers, and we need all those components to implement the same standards so that it all works well as expected. So for instance, if a particular component isn't capable of entering a low-power or active standby state, then it won't - and you can't do much about that, unless you're Apple and have a tight control over the ecosystem.
The second half of the problem comes with the software. All applications must be modern standby / S0 aware, if not, one of two things will happen: that app will keep the system awake, or the app will get suspended by the Desktop Activity Moderator (DAM). Either way, the app must be capable of running in the DRIPS phase (deepest idle runtime platform state), which rules out most Win32 apps (basically almost every app that's not on the Microsoft Store).
Finally, the reality is that most PC users don't care about modern standby regardless - and why should they, when they've all got smartphones, which handles notifications well? Also, hardly anyone does large file downloads these days, and the people who do still download, wouldn't care about doing it while on battery (and if they do, they can take manual actions to lower the power consumption, such as switching to a power saving plan and turning off the display etc).
Ultimately, most people would expect a laptop to go into a fully suspended state when the lid so closed and they're on battery, because if they're on battery the #1 concern for them would be the battery life. So most people actively seek to disable S0 and see it as a hindrance.
