Why do people think that all the cs games are heaps different?

Because they are?

play cs1.6,condition zero,cs go,cssource and maybe you will see difference :)

you need to play more then one to understand why they say that

cs16 and cscz are not that different, but css and csgo are completely different

It's mostly netcode and how the engines do certain things differently in each game, you have to play them to get a feel for it.