They often are! Even the relatively cheap (500$) kit lens of the FS5 uses exactly that to stay parfocal and prevent focus breathing. It's not perfect — there's

They often are! Even the relatively cheap (500$) kit lens of the FS5 uses exactly that to stay parfocal and prevent focus breathing. It's not perfect — there's always a lag of about 6 frames during fast zooms — but it's "good enough" for most situations (which means it's not ready for broadcast).