I don't understand why people who make these vehicle textures don't just copy real police cars and change the name to "Liberty City" instead of whatever they are copying.

Lol that's what I usually do & I encourage people to do so (Also to keep it LC friendly).

Mostly because of realism. 

@ JetSki Well Stated. I use real departments because these departments I may want to be a officer with that Department. 

Yeah but then you would need to change the whole city to base it off of it too ;)