Because it isn't at all reasonable. There is no argument to not allow root access. You don't have to use it, perhaps most users would be safer with a conventional user account, but it is not reasonable to outright deny full system right to the owner of a device since there are so many disadvantages connected to that.
My thinking is that if I have device that doesn't allow me root access, then what I have is more than likely a device designed to keep making money for the company that made it or wrote the software for it.
I'm willing to stand corrected, but I can't think of a single smartphone on the market from a reputable manufacturer that is sold with root access. If I want a smartphone I have to accept that the manufacturer will have the bootloader locked down, I don't have a choice.
I have zero experience in the android world, but a quick search tells me that Xiaomi Devices, Google Pixel Phones, OnePlus Devices, Redmi Note 4, Samsung Devices and MediaTek Devices at least are rootable, with some rules with various degrees of freedom for the procedure (in particular warranty is voided pretty much all the time when device is rooted).
Google Pixels are the few devices that enable not only to unlock the bootloader but also the ability to flash your own keys and still have secure boot together with full kernel sources availability (which is why Grapheneos only support them as far as I know).
As far as I know Mediatek (and vendors that use those chips) are usually not good with regards to GPL Compliance, which means no Lineageos if kernel sources are not available...
