My guess is because UI is a massive source of churn. Anything which is visible to a user and/or customer will be under a constant barrage of changes from custom

My guess is because UI is a massive source of churn. Anything which is visible to a user and/or customer will be under a constant barrage of changes from customers, marketing, management, etc. One reason for this is, of course, that it's actually visible to them (unlike, say, a communication layer between subsystems, which may sit there unchanged for decades). However, there's also the common perception that all software gets built from scratch, as if it were some bespoke piece of furniture. Hence anything generated, or using "off the shelf" code, will still get bogged down in change requests, not necessarily because those changes are "important" enough to justify the effort, but because "since we're building this whole thing anyway, we might as well do this part this way" (not realising that we didn't build this whole thing, and it's the changes which are the actual cost)