alt Hacker NewsCan't quite speak for UK, but in EU cases I know about its because some (popular) watches need special provisioning process for eSIMs and getting that proprietary software requires carrier to beg our favorite mega corps for provisioning software and do extra work.
It's not like phones where pSIMs were relatively brand agnostic, were back to per-hardware BS.
I have a Pixel Watch 2, it seems that only certain carriers (typically not MVNOs) can set it up with an eSIM that will present as having the same number, but any MVNO that provides eSIM setup via QR code should work to provision via the connected phone.
I can imagine Apple being less inclined to allow arbitrary eSIMs on their watches, preferring to only support the case where the eSIM is guaranteed to support the "correct" presentation.