CGT only applies on the sale of the property. As the property value rises, buy-to-let landlords will often use the equity generated to leverage another loan on another buy-to-let property, rather than sell the first property. So as they keep on collecting properties, they're not paying CGT because they aren't selling.
Especially as CGT is typically lower than income tax, so investors would prefer capital gains over rental profit. (And if their rentals are generating profit, might be tempted to buy more rentals that are not profitable but rising in capital value, so that their rental profit is lower but capital gains are higher).
You hear stories of investors who keep on acquiring and end up with dozens of buy-to-let properties and are leveraged up to the hilt.
Bradford & Bingley, which was the UK's buy-to-let mortgage specialist, was particularly hard hit by the GFC.
