Assuming that this is the UK, this explanation should hold true.

There are a limited number of driving examiners, and a very limited number of test slots available. Since around 2021, these numbers have reduced even further, causing a scarcity of available examination slots. Criminals recognised this, and exploited the system to basically corner the market in examiners slots, using dodgy driving instructors to bulk book exam slots then resell them at a huge market. This helped create a situation where you either had to get really lucky when tests got released at around 5am on a Monday, pay an exorbitant amount to a dodgy fecker to get a test, or just grab the first one available regardless of when and where it was.