Short answer. GPs cannot cope with the workload they are asked to address. So the ‘8am system’ is a way of limiting the workload.
Longer answer.
There is only so much time in a day, and so many doctors and nurses. That appointments are limited by the ‘8am system’, means there’s no more time for GPs or nurses to see people.
At least 10% of people don’t turn up for booked appointments (GP or hospital). So the public needs to not misuse an (apparently free) system. It takes two to tango.
So part of the reason these systems have been introduced is to limit as much as possible people not turning up for an appointment.
Why? If you ring at 8am and get an appointment that same day you’re very likely to turn up. If you book one for a week on Thursday the odds of you being a no show are much higher.
On any given day, there may be 4 GPs working in an average sized practice. If they see patients over 8 hours, in 2 four-hour shifts (8am-12pm, and 1-5pm), and see 6 patients an hour (10 minutes slots), then there will be a maximum of 192 patient slots per day.
But. Often 300 or more people will ring up. So 108 people will not be able to be seen.
This is one reason why patients need to use alternatives to GP and A&E provided by the NHS services.