Because it is a lot of work, not all phones support/allow rooting, you need to hide root for apps manually for them to work and there's no way to take full backups on Android phone, rooted or not (adb backup is deprecated and never actually worked at all anyway).
Meaning: every time you screw up (or get a new phone) you have to start all over, without the ability to restore a backup.
This can easily take many days worth of man-hours.
A default LineageOS installation will not be rooted. There are really less and less reasons why you'd even want a rooted phone. The killer feature used to be ad-blocking, but most ad-blockers can work without root nowadays.
The backup problem is a notorious problem with any Android phone, I don't understand what it has to do with custom ROMs.
"A default LineageOS installation" as in "a device which is officially supported through https://wiki.lineageos.org/devices/" is like that, indeed, and while most of the unofficial builds come with as -userdebug, no root, there's a high chance of encountering either a flaky build (permissive selinux) or having to fallback to a GSI build, some of them, and nowadays it means AndyCGYan's [0], come with optional root enabled.
As for backup: stock backup facilities (adb backup/restore) aren't exactly the best thing for sure, (rip Titanium Backup, Seedvault isn't even remotely there), and TWRP is broken on most of devices with modern encryption support (specifically, on Pixels). Coincidentally, Mediatek SoC based devices are the 'best' when talking about backup (as in: you can dump and clone all partitions through a built-in service mode [1]).
If you don't want root, then don't root it; that's separate from replacing the ROM.
> there's no way to take full backups on Android phone, rooted or not
Er, no? On the contrary, root is exactly how you get full backups on Android - with, I grant, the exception of whatever Android calls their secure element / TPM, which I've only ever seen affect 2FA apps.
yeah, good luck restoring full userdata and metadata partitions on a rooted pixel 4 and newer: you definitely can do that (and bypass TPM), but since there's no working twrp, there's no exactly easy way to dump/restore partition image with dd anymore.
? https://twrp.me/Devices/Google/ says there are official TWRP ports for pixel 4 and 5. I'm surprised at the lack of ports for newer versions; not sure what went wrong there. Yes, if you somehow get a phone without a working TWRP then you're going to have a bad time.
Because it costs them money. And sometimes you can in fact give feedback.
Also, in boardrooms when sales are discussed, it will raise more eyebrows when there are lots of returned items. In contrast, when only the sales numbers are low, typically the response is to increase consumer-unfriendly measures.
