Why does any Dr or receptionist think their time is more valuable than mine?
It's not GPs who are inconvenienced when you are late; it's other patients. GPs typically see 15 patients back to back, at 10 minute intervals, in each half-day surgery , immediately followed by home visits. If each patient is 5 minutes late/runs 5 minutes over, by the end of the surgery, the GP will be running 75 minutes behind, and the poor patients at the end of the surgery- and those awaiting home visits- have a massive wait.
Obviously it's not common for every patient to be late, but it's very common for every patient to require more than 10 minutes. And late patients still expect their full appointment time - which means everyone else in the waiting room is inconvenienced. Your question should really be why you think your time is more valuable than that of other patients?
GPs could opt to see fewer patients, of course, but then it would be even harder to get an appointment. There is no slack in a GP day at all - on top of the things already mentioned, they will have phone calls to make, tests & letters to review and prescriptions to sign.