They are, but they don't have to move a ship faster that light.
Hard to do, because an active fusion reactor would generate a lot of heat, on top of the hull's interior being warm.
according to the research ive done space is 3 degrees kelvin
step one
partially lower control rods, power down systems like your recharging alcubierre drive capacitor or your combat maneuvering thruster array
step two
partially cool the part of the ship facing the thing you want to hide from, vent in the direction opposite of it
step three
remember how i said partially cool?
reduce cooling loads by cloaking as something that fits the environment, like a few distant stars or perhaps a small meteoroid
stealth is a far more realistic technology than the alcubierre drive. this is coming from someone who wants more science done with it, but... well, the alcubierre drive - at least, in simulations and calculations, likes to collect particles while it jumps and discharge them when it stops.
in a conical area in the direction the ship was travelling
at velocities high enough to destroy planets or perhaps stars, depending on how many particles got scooped
this isn't even mentioning other methods of stealth like
this, or this.Maybe you make yourself invisible to heat. Cool, have you also plugged up the exhaust so that they can't pick up your engine emissions? What if the scanners don't just search for heat or radar, but also for spatial distortions as a result of using the Alcubierre drive? What if the scanners are just that good that they can spot a ship visually anyway?
the alcubierre drive doesn't leave lasting after effects in its environment, at least not in the fabric of time space itself. again, it's loving impractical as stuff because it's a WMD beyond the likes of anything humanity has ever created - it's the MOAB where the tsar bomba is nothing more than a cherry bomb. sure, you won't be stealthy when you're in an FTL jump but you can't get intercepted when you're superluminal.
also it's worth noting that the advanced massive scanner arrays that have all that wonderful space and lack of weight limits on the ground for gigantic systems wouldn't work in a functioning ship.