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.
"Easy" is relative. "Evicting" an airbnb'er once the term is up will always be orders of magnitude easier than kicking out a long-term tenant who regards that as their actual home. There's not even anything necessarily wrong with this! The easiest way to address the issue is literally to slash any remaining red-tape that's making things difficult for those who would want to AirBnb these properties out, while managing the resulting side-effects (including the plausible effect on the long-term rentals market) by levying a special fee if necessary.
As a bonus, easy AirBnb rental provides an alternative for some who might otherwise want a permanent holiday home.
If they can't afford a 'vacation home' without short term rentals then it should be sold and provide housing year-round and build equity for a new homeowner.
That does not mean hotels would stop being built, because developers and hotel operators are optimizing for a different kind of value. But it is more than enough reason to oppose turning even a single home into a de facto short-term hotel.
The reality is not simply that people are monetizing unused space. The ability to rent homes short term encourages people to buy more second homes than they otherwise would, because the income offsets the cost. It also encourages investment in housing that would otherwise remain in the long-term market, since owners know unused time can be monetized. Some buyers purchase properties specifically to operate them as Airbnbs, and many landlords convert homes that once served monthly or yearly tenants into short-term rentals because they can charge more, adjust rates freely, and often earn more overall.
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.)