I imagine its mainly a matter of 1) expertise, and 2) priorities.

1. The detectives who are actually good at solving crimes get burdened with hundreds of cases, then suffer from burnout and retire early like a lot of professions nowadays. Then the people who come to replace them have no experience, have to learn from scratch, and the cycle repeats.
2. Solving murders (especially when it involves poor people getting murdered) isn't profitable, and isn't prioritized as much as revenue-generating activities like drug busts, traffic ticketing, and getting poor and homeless people into the court and prison system. Like its probably <1% of cops of any given larger city that work on and know how to solve crimes like murder or theft. Cops are the hired goons of your city's capitalists, protecting their property is what they're paid to do.

My car got broken into when I was in highschool, and I learned the lesson that US cops aren't going to help you, and probably don't know how to solve even simple crimes like petty theft. It gains them nothing to help you, and that's not what they're being paid to do.