Why are Mov Spd quints so expensive?

Currently priced at 2050gp which is double the price of flat penetration runes (1025gp).

Now I know mov spd is a valuable stat that much ppl don't give credit for, its hidden power can determine a lot. But by buying these quints you sacrifice offensive and defensive power that opposing play most often take.

I wouldn't mind paying 1550gp for 1 mov spd quint cause mov spd is rare in some champion builds but 2050 is to much for a small stat that I only find viable on a handful of melee champs.