Starting in 2013, the FCC began regulating the cell phone signal booster industry in order to improve performance across the board and to prevent unregulated signal boosters from interfering with cell towers.
One of the stipulations is to sell cell phone boosters as complete kits with the amplifiers, antennas, cabling, and other necessary parts for all consumer and commercial products.
Learn more about this and other FCC signal booster regulations: Why cell phone signal boosters are sold as a complete kit.