As a US citizen I traveled in and out of the US 4-6 times/year between 1998 and 2018, and was never subjected to an interview. Of course, I'm super white and have an Anglo-Saxon last name.
However, in the years following September 11, 2001, my whole family has been subjected to extra screening on a number of occasions - probably because our passports were covered with stamps from Indonesia and Egypt.
This never happened to me while I was traveling alone, but both my husband and son got extra screening on occasions when either traveling alone or with one or more family members.
The evidence suggests that a variety of factors (name, gender, age, and the assortment of countries you travel to/reside in) can affect the level of scrutiny you receive. As little old white lady living in Hawaii and traveling very little, I'm probably home free now. While that's nice for me personally, I have tons of sympathy for people whose profile is different. In recent years I haven't witnessed anything terrible, but until 2010 or so there were occasions when I witnessed people taken aside for extra screening in ways that seemed unfair and racially charged.