You have to lift the cab off to do a LOT of things on the modern pickups.. even the gassers. And it isn't really so much so that you always HAVE to do it, but it becomes the lesser of two evils. Lifting the cab, while it certainly looks daunting, it really isn't. For the guys that do them all the time, they can have the cab off in usually two hours or less. Then, once at that point, pretty much everything else engine-related on the top end is fairly easy because it becomes almost the same as having the engine on a stand.
One of the reasons for all this cab lifting is simply because they've moved the engine compartment back and under the cowl, tight against the firewall. Add in that they've made the trucks sit up higher, especially the 4WD ones (and this is of course made much worse when owners add on the Teeny Peeny Package) and it becomes virtually impossible to reach anything on the back side of the engine. And there is no access panel ("doghouse") like some vans have.
Remember, body-on-frame pickups were BUILT with a completed chassis (engine-trans-axles-brakes-etc.) all bolted together on the frame, and the body is lowered in two sections (bed-cab) down on to the chassis. So it isn't difficult to reverse the process. You do, however, need a lift. No way around that (although a crane, etc. can certainly be built).