Physical access is just such a rich attack surface that keeping your computer away from malicious actors is the right and proper solution.
An extreme example a pentester imparted to me once was, if someone could spend sufficient time alone with my laptop, they could remove my hard drive and insert it into an identical laptop with a hardware or firmware backdoor preinstalled. We were discussing nation-state adversaries, but the general principle applies.
Another example is attacks on encrypted drives (so-called "evil maid" attacks). If a computer is booted and the drive is decrypted, an attacker with physical access could open the computer, remove the RAM, and download it's contents, thereby stealing the encryption key. If the computer is powered down, it's still vulnerable to other attacks; enrypted drives necessarily have cleartext code for accepting the password & decrypting the drive. You could modify this code to log the decryption key, or broadcast it over your device's radios.
There's also the classic Windows "sticky key" exploit, where you replace the sticky key binary with a program that gives you administrator access, reboot the computer, and then activate sticky keys.
You could install a keystroke logger. You could install a device to record monitor output. You could log network traffic.
I've yet to find a kiosk environment that I couldn't break out of. Once I was able to break out of a scanning kiosk environment, and into a Windows desktop, by turning the quality settings all the way up and crashing the kiosk. That was one of the more difficult examples; most of the time all you need is to find a way to right-click. (I had the proper authority to investigate these kiosks.)
The point is that the list goes on.
It is true, as you say, that there has been progress in implementing mitigations, and that there are people who care deeply about these issues. A counterexample might be SIM cards, TPMs, and other HSMs. These systems are able to provide better guarantees by encapsulating their peripherals and being willing to self destruct. But that could describe a cell phone, tablet a laptop, too.
Maybe in the future this "law" won't be so hard and fast.