Not all devices have a screen or an easy method to input a TRR URL. So even if a vendor has the best intentions, the effort needed to make DoH configurable could be more than what can be justified for a "niche feature".

I believe there are ideas for a network-level autodiscovery of DoH servers, analogous to regular DNS. That would solve the problem, but I can't see how it would get buy-in from the DoH crowd, as it would tie DNS to the network again - and it would still have to be implemented by every single device.

Of course all of this is moot if the vendor actively wants to prevent you from intercepting DNS.