Even if we buy the exotic premise that investors would buy houses and leave them empty - generating zero rental income for these greed driven people - as an investment, we still have to explain why they would only do this with new housing stock.
Why can't you do the same trick with existing housing?
Where I live the housing prices are expected to go up roughly 15% this year. So someone buying a house for 600k can expect to sell it for 690k making 90k. That is 7500 USD a month. If you wanted to make that much money from rent you would have to squeeze 4 or 5 tenants into the property. It is simply not worth the hassle trying to rent buildings out when house prices go up at such a high rate
Make $7500/month from house value increase, and also, say, $4k from rent.
This increases the resale value since the house is generating income to the buyer.
It's not either/or, which is why I'd assume the parent poster called it weird. That investor could make the appreciation and of top of it could make the rental income. So it is weird, from their perspective, to leave it empty. If they're flipping real estate for profit (as opposed to buying a home to live) why reject the extra income.
Also, selling a rental property with a solid history of rental income tends to be better than one with a history of sitting empty. Although that probably doesn't apply to mansions.