Partially, partially because the sites use quite a few plugins and I find it more convenient to use WordPress for them than to recode everything from scratch to work with a new system.
I actually listed all the things I'd need to recreate with a migration, and it's basically:
1. The entire account system
2. Plus the admin dashboard
3. Post/content editor
4. Media library
5. Paid subscriptions setup
6. Social media integrations
7. Apple News integration
8. Permalinks/friendly URLs
9. Any systems WordPress has for data sanitisation
10. Plus the challenge of either matching the original URL setup or flawlessly redirecting about 20,000 pages.
Also budget stuff. Most companies I've worked for don't spend a lot of time building their own site/blog, and wouldn't budget for an entirely custom CMS build or migration.
