Yes, in a lot of cases it has become a default without much thought behind why a "Big 4" rather than "respected professional services firm" background is needed. Other times, the job requires a certain workstyle and culture that is characteristic of the "Big 4" (i.e. it's shorthand for "professional, corporate attitude, structured worker, steady worker, used to playing politics without upsetting the applecart"). Other times, it's simply a matter of brand - the employer is only familiar with the Big 4 and wants candidates with a branded background they can trust and where they can be reasonably sure what they're going to get. Finally, there are the expectations of the employer's own clients - if they are traditional in their approach to consultants they consider credible, the employer may need a "Big 4" name on your CV before they can successfully sell you into projects.