Why must infertile women adopt? (Please see details)

A common thing I hear and read from other people is that women who are infertile and spend thousands of dollars and many years trying to get pregnant should just adopt.

What I want to know is: why? Why should these women be more greatly pressured to adopt than, say, a woman who has six children and conceived them all naturally?

I am especially interested to hear from other people who were not conceived or who did not conceive the natural way but I do want to hear from everyone on this subject.