why it behaves this way as an academic question, though any explanation might become out of date in a future version.
It behaves this way because the developers need a consistent way to identify disks, and they are not trying to generally solve a difficult problem for the benefit of all users everywhere, but rather to solve the problem for their limited use case. iXsystems develops TrueNAS for their TrueNAS hardware, hard drives have a label with the serial number on top, industry practices often include marking those serial numbers on the drive tray for front visibility, and that identifier is typically easily and reliably accessible from inside the OS. You need an identifier that can uniquely identify the drive, so that the NAS can identify when disks move around, and that the NAS can understand the inventory of available drives and not make catastrophic mistakes like thinking that /dev/da34 and /dev/da62 are different devices, when one is actually a secondary SAS appearance of the other. Because if it thinks they're separate devices, and it's used one, and thinks the other is free, horrible things *could* happen to your pool if it started to blank da62, killing da34 which is already part of a pool.
iXsystems is a small company, and there is neither a huge amount of idle engineering talent sitting around just waiting to tackle intractable issues like these in software just for the benefit of people wanting "alternative choices", nor are there good general solutions for these problems, which most frequently manifests in the form of people trying to hook up USB drives using bridges which all have the same serial number hard-coded.
iXsystems is focused instead on making a NAS for its hardware, and to the extent that you can create a box similar to theirs and submit to their appliance-based design, they have made their work available to you to enjoy for free.
any explanation might become out of date in a future version.
Unlikely, unless some BETTER identifier magically apparates. Generally, as members of the community, it's helpful to remember that this is a commercial PRODUCT and is made available for free. The reason for this is because we are beta-testers, bug-finders, and anything that is beyond what paying iX customers would need is dubious; the product is effectively out there on an "as-is" basis (from the viewpoint of your by-id request) but they are definitely interested in bugs and feature requests that could improve the product that they sell.
This doesn't mean you can't open a Jira ticket and prove me wrong. I'm happy to be wrong. But I've been here more than a decade and seen this for a long time. You are unlikely to get a detailed answer from iX if they reject your feature request, so I'm trying to do the courtesy of giving you that glimpse into the "why" of it.
