The full explanation would take a wall of text (and still let you unconvinced because you feel entitled to do as you please, probably).

Super-condensed version: civilian flight are a pretty difficult "product" to handle efficiently. Price increases until 1 minute before closing the airplane doors, then falls to zero. On top of that, the product "provider" also needs its own product in order to move personnel and technicians all over the globe, but of course they cannot just cannibalize their own products beyond the point of profitability.

Plus they have to handle rebookings and passenger protection in cases like delays, sudden airport close-down and so on. (Have you ever been on a waiting list, btw?).

All this is pretty complicated to manage already, so they need to exert as much control as possible on yield and occupancy.

TL;DR: a flight is not a bus ride. So if you just decide to cut it short the airline will try to reuse your vacant space for whatever reason.