IAS 40

The short but glib answer would be because the standard says this classification as investment property is not allowed.A more detailed reason may be that it would be difficult to determine to what extent a property was being truly held for investment purposes and to what extent it was being used as a reward to employees for services to the company.This could be achieved by charging a below market rent to employees renting said property.In other words it would be difficult establish the degree to which the relationship between tenant and landlord was conducted at “arms length”.For similar reasons tax jurisdictions tend to have legislation establishing deemed benefit of renting from employer when calculating an employees tax liability.
In my view this is what makes Accounting interesting.The fact that we can have different opinions about the appropriateness of an Accounting treatment.However, when all is said and done we must follow international accounting standards.