I think what you saw as "cabinet without head", is actually performers using rack mount pre-amp, effects & amp module. Very much like pro-audio stuffs normally packaged into 19" flight case for convenience.
Major brands (marshall, mesa, SWR....) do produce 'high end' rack mount equipments. These are normally placed way behind or the side of stage.
Bass performer more often uses such config than guitarist, as their amps are very heavey and powerful.
Combos are more often used by guitarist as their amps are normally not that high power, as combos are more affected by rattle problem caused by heavy bass sound. The guitar voice is more mid range, less rattle, so this problem is not common. A combo is much more convenient and light than rack equipments.