Short term rentals are better on that score: no one sensible forms a long-term expectation that they're going to live in an Airbnb that they've rented for a few days. (If you think short-term rentals are "bad" for the long-term market or have negative side-effects on the neighborhood, then tax them to manage that tradeoff. But banning them altogether is unconscionable and just leads to houses sitting empty and unused.)
Although the easiest route (no-fault eviction) is being abolished soon, wanting to sell a property remains a valid reason to evict.
Some people just buy property as a speculative investment.
I do not think it is the main cause of shortages in London - that is people buying holiday homes, which are often large and centrally located. London provides a lot (restaurants, nightclubs, gambling, prostitution, financial services...) that attracts people with a lot of money to spend from all over the world.