imagine a large factory or 6 level building sitting empty for decades.

More likely the property speculators are not interested in buying it, and the owners (if still extant) not interested in selling it.

I know of two enormous blocks of flats built as Royal Naval shore accommodation... only a few years later the Government of the day closed the Navy Base they served. Forward planning, eh?

Some bunch of London-based speculators bought both blocks, and eventually converted one into "new" residential flats. They were slow to sell, and I suppose the best went as second-homes for over-paid City types. The other remains stripped to frame and floor-slabs decades later.

I thought it a classic case of "investment by dereliction": leaving the property to go to rack and ruin until home prices have risen enough to more than cover the capital and conversion costs, while spending no money on the site in the meantime; but it looks as if the spivs have abandoned it. They've probably recouped the cost of buying and stripping both buildings, and refitting one, on the sales of just that one block's flats.

There are a bunch of building that have toxic contamination or asbestos or structural issues that would need to be fixed before the building can be used or even torn down. Sometimes it’s less expensive to pay taxes on a property than it would be to fix the problems 🤷‍♂

might be full of asbestos or have had some kind of oil/chemical leak that needs to be dug up