Why does USCIS take such a long time to process an application for?
Asking a "why" question about the government will likely never get you the answer you want. As a special request, I will humbly speculate as to the reason why a government agency takes X period of time to do Y. As a general answer, which can probably be applied to any government agency, the agency has a finite number of resources (only so many dedicated staff members) who must therefore (thoroughly) review and approve/deny/request more evidence with regards to each application/petition received. The three months it takes for USCIS to process an EAD application for F1 students on STEM/OPT is actually on par with all other EAD application types. In fact, USCIS, for the past few years, has been fairly consistent and reliable w/ the processing times for EADs in general. USCIS employs a dedicated team of officers to review certain types of applications/petitions. I can only speculate that because each officer has a massive number of applications/petitions to review, it takes approximately three months for the life cycle of an F-1 student's EAD application to be complete, assuming all the required documentation has been submitted. IMHO, three months is remarkably fast.