But you can disable Secure Boot and boot whatever OS you want, so unless there's some other hardware gotcha it's not like someone couldn't get Linux running if they wanted to put the time in (which is a big if, considering there's no UEFI-ish helper like on the Windows ARM devices).
There are quite literally millions of ARM devices out there that will never have Linux support, and millions more are being produced each year.
When it comes to ARM SoCs, Linux requires vendor support to get it running. If you want mainline kernel support, that requires even more work that many vendors just aren't providing.
A locked bootloader is just one issue to overcome for Linux support. A lot of the real issues come down to the lack of an enumerable bus on ARM SoCs, along with a lack of drivers.
Without vendor support from Apple to support Linux, these devices will be like the millions of iPhones and iPads that don't run Linux and will never run Linux.
Most ARM SoCs that are sold explicitly as mini Linux computers also have this problem. Many of them are stuck on old kernel forks, because vendors didn't give the proper support their SoCs needed to run a mainline Linux kernel.
tl;dr: For Linux to be a viable option on Apple's SoCs, Apple needs to put in a lot of work to explicitly support Linux. Without that vendor support, you will never be able to download a Linux ISO and install it like you can on an x86 Mac.
https://support.apple.com/guide/mac-help/macos-recovery-a-ma...