There are charter operators who do that kind of thing, such as for holiday destinations and religious pilgrimages, as well as cargo airlines. There are also "w

There are charter operators who do that kind of thing, such as for holiday destinations and religious pilgrimages, as well as cargo airlines. There are also "wet leases" where a provider has aircraft and crew packages you can deploy for a fee, as well as dry leases where it is just the aircraft. However as other responders mentioned, aircraft logistics are complicated. Unless you operate virtually identical aircraft (the Southwest, Ryanair etc model) then you need both pilots and cabin crew that are certified on the aircraft model. Plus you need appropriate maintenance staff, baggage handlers, gate staff etc. It is just too complicated to pull off logistically. That isn't to say major airlines don't have the ability to respond on-demand. They do have spare aircraft around, and have ones that can be chartered. For example that is how sports team travel around the US. Also be aware that airlines have a typical load factor (how many of the seats they sold on each flight) of around 85%. There isn't that much slack in the system. 25 years ago, load factors of 65% were considered the norm. These higher load factors are also why the frequent flyer programs are so much more stingy now - they have more paying customers to fill more seats.