The cars are smaller more cramped than they were 40 years ago and are jam-packed full of P25 trunked 700-800 MHz radio equipment, a computer terminal, scanner, and whatnot.
I think this is often overlooked.
There's a lot of equipment in these cars. Cramming in a CB when that space can be used for more important items would be likely.
Antenna is the other challenge. Antenna placement is important to a properly functioning radio would prioritize their LMR radio over a CB. We all have read on this site how important proper ground plane is to antenna function, and with a CB antenna being pretty low on the list of priorities, it's not going to function well.
Officers have to listen to a lot of radio traffic, watch their terminal, driving, everything else. Having to listen to the excrement that gets spewed out on CB is going to result in the radio getting turned down, squelched up high, RF gain low, or even turned off. Next time the car gets refreshed, the likelihood of a CB getting reinstalled is going to drop even further.
The number of rando-citizens that actually have a functional CB in their vehicle is pretty low in most areas. The number of those people that do have one, and have a properly functioning antenna is even going to be smaller. So adding a CB plus an antenna to a patrol car is going to benefit a -very- small segment of the population that would probably pick up their cell phone and dial 911 first.
Not a good return on investment. No a good use of real estate in the vehicle. Not a good use of antenna space on the vehicle, either.
When just about everyone has a cell phone, the benefit of a CB is further reduced.
I think the last CB we had in a patrol car was back in the late 1990's/early 2000's (pretty sure it as in a late 1990's Suburban). That was a guy that covered a remote location. He never really used it, but it was there. After the early 2000's, it was never replaced.
CB isn't what it was 40 years ago. Those days are long gone. Agencies wasting time listening to the total hash that's on CB is unrealistic. Only place I've seen CB in use by public safety is on the commercial enforcement vehicles the CHP has. No idea if the CB's are actually turned on, or if they are even listening to channel 9. More than likely they are on 19 listening to the trucks.
Hopefully no one would rely on CB channel 9 in a true emergency. So many better options.