First, as shitty as it is to not participate in as a customer, tipping culture is, for the most part, optional. When a server indirectly asks me to tip as a customer, I could easily hit the custom tip button and enter 0.00$. That would be shitty on my part as I would be reducing the income of waitstaff who rely on tips. If I tip, I now have a few dollars less, and the waitstaff have a more livable wage. If a police officer asks me to get on the ground with my hands on the back of my head, I don't have much of a choice. If I do, the police officer will likely arrest me, and this compliance is only coming at the threat of what happens if I don't. If I refuse, then the police officer could shoot me (if he deems me a sufficient threat), taze me, pepper spray me, or otherwise physically force me to the ground and possibly injure me. Further, I could get in significant legal trouble for not following the orders, most often in the form of resisting arrest, or possibly getting charged with assaulting a police officer if I act in self defense, regardless if I act within the law. This problem here lies in the fact that there is hierarchic authority that a police officer has which waitstaff lack.

Second, there is something that servers can do to make the system better outside of participating in the system laid out by their boss. While not easy, and with some risks attached, waitstaff can unionize and demand better pay, such that no tips are needed. Obviously, it isn't super likely that the union would remove tips because waitstaff like their tips, but this act will fix one part of the system, being the part that they are not paid living wages before tips. While unlikely, widespread unionization could cause people to want to tip less knowing that waitstaff are able to subsist on wages alone and therefore impact tipping culture.

Cops don't have this ability. I'd argue that police unions are not the same as a typical labor union. Like a normal union, they provide the workers protection from being fired, and have a positive impact on wages. Unlike a police union, police officers are called to break up the strikes of labor unions. If the police union went on strike, the only theoretical way for their employer, the state, to break it up would be using another militaristic arm of the state, be it the state reserve militia, if it exists in that state, or the military in other cases. Unlike calling the police, there is significant political capital being expended when doing this.

Another point to consider with that is which cops are fired, what leads to that happening, the impact of it, and how they are protected. Often, it's "bad cops" rather than good cops, though both is possible. The union often steps in to protect even the worst cops from being fired. The impact of a bad cop is significantly more harmful than a bad server. A bad cop is violent, often kills or maims people, and terrorizes communities. A bad server might spit in my food, let it get cold/warm, or not deliver it at all. Short of physically hitting me (which a union will not protect them for), the most harmful thing they could do is steal my credit card details. Bad cops are fully and legally able to do much worse through civil asset forfeiture.

Lastly, and most importantly, the context of the system is vastly different. I'd argue the most harmful system that is held up by a server working a job isn't tipping culture, but wage labor (and capitalism) itself. Just like police, anyone participating in this system cannot fix it by participating in it. Unlike police, those participating in wage labor lack the power to directly reinforce it through violent action because they lack the state's monopoly on violence that the police lovingly wield. Any harm done by a person reinforcing this system can be offset by various acts, such as creating and participating in labor unions, creating co-ops, protesting and agitating for socialism, etc.

Police, on the other hand, not only indirectly reinforce this system by being payed wages, but they also directly reinforce this system by making it difficult to combat wage labor by breaking up strikes, protecting private property, terrorizing and killing protesters, killing organizers, etc.

Worse yet, police also directly support the hierarchic structure of the state, an unjust hierarchy, and the unjust hierarchies of white supremacy, patriarchy, ableism and cisheteronormativity. Police have always been the arm of the state that has had their literal boot on the neck of black people, suffocating their communities. When the police are not the ones to harm these communities, they often don't do that much to prevent it from happening, or prevent it from happening in the future. Let's not forget that about 50% of those killed by cops have some sort of disability, or their historic violence against LGBTQ people, and how LGBTQ rights were only taken by clashing with the cops at Stonewall and demanding rights. While police aren't the sole people upholding these hierarchies, they are one of the most arms of the state doing it.