The revolving door as it's known. That's part of it. Another is simply the lack of in-house talent, largely due to poor pay and conditions. It's a self-fulfilling prophecy to a certain extent. I'd love to work for NHS digital and make a difference, but all the interesting work is contracted out, so they can't keep the staff who are capable of building themselves. Also the recruitment process is terrible.
Take a look at this job posting for example: https://www.jobs.nhs.uk/candidate/jobadvert/C9175-26-0093 .
The role is more aligned with IT/Data as obvious by the fact that the main skill requirement is SQL.