The massive difference between a car and a great big plane is...... You drive your car! All cars are virtually the same inside and can be driven by anyone who holds a valid driving licence.
Boeing does not have spare planes lying around in a desert, the ones that are stored are owned by other companies and the majority would require some sort of maintenance to reactivate. By the time a plane is sourced, contracts are signed with the various owners, pilots are sourced, trained and employed, cabin crew are sourced trained and employed your plane is fixed and there is no point getting one plane out to an operator that will most likely have their own jet fixed by the time all this rigmarole is sorted (and a lot of money has been spent)
Even ACMI operators require notice to operate flights on behalf of a third party, and a minority of passengers will still most likely be unhappy because they "didn't pay to fly on this plane".
Better to compensate the said airline in other ways, which that airline can then use to compensate the passengers.