They take a loss on earning in the hope the capital gains covers it, the cgt was also halved in 2000 to make it more worthwhile.
you might lose 2-3% by reducing your taxable income via taking a loss but it allows you to borrow more and if you think property doubles every lets say 10 years then you make10% a year minues 2-3% loss = gains of 7% year on borrowed money.
After ten years you turn 1 mill into 2 mill, you only pay tax on 500k of that so essentially you make 700k off every mill minus your 10x 2.5% loss = 75% return on monies borrowed every ten years
Just quick back of the envelope figures but for the last 25 years its been made that if you dont borrow and arent aiming for capital gains your a fool, yield is for grandmas.
Hence we have massive public and private debt and asset bubbles
