Stick shaker speeds have to be relatively near the stall in order to allow reasonable manoeuvring capability at typical operational speeds. If they were set too far from the stall any moderate turn in the takeoff or landing configs might set off the shaker.
Also, its supposed to represent aerodynamic warning for a naturally stalling wing, which doesn't occur a huge amount before the stall.
Finally, psychologically, if shaker was at 1.20Vs (silly example) and was going off 'all the time' as a result, people would start ignoring it as a nuisance - a 'Chicken Little' effect.