You are basically telling god... that you know what is best, more than god does
By praying, you are saying...
"Hey god, I know you don't want to give me a new car, but I want it anyway"
"Screw your perfect/divine plan god, I want a new car... me me me me me me, it's all about me"
If it is god's will... why are you trying to change god's will?
Do you really think you know better than god does?
Or, are people just selfish and don't care what god has planned?
I got your point and it is very true. Some people to call their God when they have their wish only.
There are something that they want to have or want to happen, maybe we should accept that in some time we became selfish.