Part of what’s going on here is that clerks make much less money than if they had gone straight to firms. To be induced to apply to clerk, some law students need to know that there’s a big pay day at the end. Also consider that SCOTUS clerks need to spend at least a year clerking for a lower court judge beforehand, again at pay that is way under what they’d be making in a firm.

Of course, some clerks don’t need the monetary motivation because clerking itself is reward enough, but for many law students it only makes sense if they can recapture earnings through a clerkship bonus.

Source: I’m a former lawyer and dated a lawyer who clerked for an appellate court and interviewed for a SCOTUS clerkship.