Agencies. WordPress is a commoditised product that puts them on the same level as everybody else. Whereas a bespoke CMS locks clients in, whilst providing a “unique client experience”. It’s “not invented here” syndrome as suffered by business owners.

It’s total bullshit of course, but I think they’re the people who sustain the myth. It’s how you end up with some tiny company that very rarely changes their website having a bespoke CMS.