Its mainly because people want gil, dont matter if its 50% less than what they couldve got in 3 days, fast gil.
A funny story about an undercutter, i was trying to sell my kingly whisker for 10 mil, there was always 1 other guy that undercut me, not by a huge amount, but i noticed eventually he would undercut me no matter how low i went, so i tried putting my whisker at 2.5 mil, an sure enough he put his for 2.3mil. So i bought his whisker and repriced both at 10 mil and they sold within a week.
Just shows some people dont want the most gil, they want the fastest gil.