Because these things are unpredictable and can change on the day. Say the first patient on the list is having a procedure expected to last 4 hours, but they come in and they’ve accidentally taken their blood thinner, or they mention they are a bit breathless and staff do an ECG and their heart is suddenly in a funny rhythm, or for whatever reason their operation has to be cancelled because it’s not safe for them to have it. If all the other patients aren’t coming in until later, you end up with a slot going wasted and an empty theatre and a team twiddling their thumbs until the next patient arrives, which isn’t an effective use of resources.
I appreciate it’s anxiety provoking, uncomfortable, boring etc etc to be kept waiting, and it might not always seem like it on the other side but there are generally good reasons why lists are done in a particular order.