I would not be exclusive here. I saw some forex houses which offer BTC>Fiat pairs.
I will say in crypto trading you mostly always have some value. At any time you can withdraw Bitcoin (or any other coin/token) or fiat.
On Forex, you buy appreciation/depreciation of some currency. For example, you can create a sell EUR to USD order, even if you haven't EUR. perhaps it is more obvious when you place an order to sell sugar even though you don't have it or have never bought it.
it is more like gambling where you predict in which direction a certain market will move, and you bet on that.