Actually they sort of can't.
The Fel Horde is dying. I mean, make no mistake, they are powerful, but the influence of fel magic makes it practically impossible to achieve a good birth rate. This is I suspect part of the reason the Fel Horde retreated into Shadowmoon Valley; while they've a lot of troops they lack the infrastructure needed to actually strike, so that's what they're setting up in Shadowmoon Valley with demon support.
But even then, the Fel Horde can make good on its losses once, or twice, but after that it's spent utterly. And between the Mag'har, the Draenei and the Alliance it can't make it stick. It'd be diplomatically very difficult, but if need be we'd join forces to bottle up Shadowmoon Valley. Even if we're all broken individually and unable to mount a defense or a counter attack against the Burning Legion in Shadowmoon Valley, we'd have the spare troop capacity between us to keep them there.
And they'd have much more difficulty replacing losses than we do over the longer span of time.