you are reserving a spot in a queue. it doesn't guarantee that your spot will be first when you get there. if you go early or on time you might not be the first person in line, and if you don't show up even the one minute or two they are waiting for you adds up during the day. worse if you show up late you'll just make the queue longer. the walk-ins always consider how many people are already waiting, and if you are not there (but are still reserving your ghost spot due to arriving late) someone that might've left doesn't.

the average time of a visit (that determines how many visits are given per day) does not dynamically change based on predictable past patient data, and both the doctor and the insurance are incentivized to underestimate visit times, so you'll always face a line that is growing because the service rate doesn't match the arrival rate of reserves+walk-ins. visit times also vary, and even if everything else was perfect, due to randomness alone you would still have to wait for some if not most days depending on the distribution of the visit durations.

all and all it's multiple factors combined to make visits as shitty as possible.