The kits contain other necessary items, like brake lines, mounting hardware, and actuators. Hub and brake rotor are also a single unit on the kits I've looked at. You'd have to find a truck that uses bearings that fit the trailer spindles, and hubs that accommodate the length of the spindle. You would need to figure out a way to mount the calipers too.
In the end the kit is probably cheaper.
2006 Silverado 1500HD Crew Cab 2WD 6.0L 3.73 8600 GVWR
2018 Coachmen Catalina Legacy Edition 223RBS
1991 Palomino Filly PUP