Beat cops aren't really empowered to do what a normal person would consider an "investigation". (their "investigations" are stopping people and asking them questions) They write citations and reports of incidents they directly witness to and statements by people they directly talk to.

No way in hell 2 patrol cops are gonna do all the paperwork or are encouraged to go track down the various video footage to find out who hit you. They might refer it to a detective. Who wont action something that will likely result in a misdemeanor offense or very light straight to probation felony at best. Especially when you consider detectives workloads. It makes a certain degree of sense when considering prioritization. The detectives supervisor would probably lambast them for deciding to spend the half day to track all that down when more serious crime could have been pursued instead.

Meanwhile the detective has way more incentives in his social group to prioritize finding the person who hit the SFPD detective. The detectives supervisor might even tell them to drop everything and prioritize to help internal office politics!