The main reason to buy those domains is to spend as little money as possible. Someone at gov.uk is a cheapskate or works with a really limited budget.
I guess there’s got to be some incentive for teams to not register hundreds of subdomains, but if they are turning to 3rd party TLDs to host government assets, I’d say something has gone wrong.
Isn't this basically the very definition of "shadow IT"?
Then you find that some prod[-ish] service relies on an external provider that's not listed as a provider in any of your org's financial systems, is paid by some random internal person's credit card and expensed back ... then that internal person leaves, and N weeks/months later all hell breaks loose when your service falls over and no-one knows why.
.gov.uk domains are registered through commercial registrars (who charge a fee), much like .co.uk domain names. The main difference being that you need to be an eligible organisation to register a .gov.uk domain name.
There are likely procedures in place and security checks and people involved in gov.uk domain control, that are a non issue if the team owns the tld, which makes internal testing easier.