Because currently there is not a level playing field and rich overseas residents individuals treat the UK like a tax haven and are not subject to tax on transactions that a UK resident would be subject to tax on.
Currently overseas resident individuals can buy and sell UK properties and avoid UK tax.
It is possible for them to avoid stamp duty when they buy them (by using overseas structures) and they are not subject to capital gains tax when they sell even if it is not their main residence.
The tax laws should be changed to:
a) make it impossible to avoid stamp duty on acquisition
b) make the sale of UK real estate subject to CGT even for non residents (like in Australia). If the individual owns no other worldwide property then the principle private exemption could apply.