1.5 million housing units != 1.5 million housing units for affordable rent

London literally has tens of thousands of dwellings sitting empty. Many are investment properties and will be sold multiple times without ever being lived in.

It's easy to make money at the high end from rising property values without the hassle of dealing with tenants.

It's easy to make money at the low end by packing as many tenants as possible into small properties and spending as little as possible on maintenance. There's occasionally some legal hassle with evictions, but generally for landlords it's a seller's market, with minimal regulation and enforcement.