Many rent their properties while they wait for a sale to go through. In France it can take years to shift a property - I have even seen special loans on offer for a duration of up to three years if you are in a chain. In such cases, short term rental is a perfect solution for the owner.
In Paris I have seen agents renting properties for sale as their main business. When owners no longer can commit to long term rentals, the short term market is perfect.