Why are so many cars look alike but their manufacturers are different?

like, are they copying from each other? let's say the chrysler 300c, looks like a rolls royce ghost and phantom. the 1970 dodge challenger is almost exactly like the plymouth barracuda except the headlights are different. the 1970 plymouth superbird is also like the 1970 dodge daytona. the acura nsx is exactly the same as the honda nsx. the holden monaro is the same as the pontiac GTO. and lastly, the lincoln town car, mercury grand marquis and the ford crown victoria all look pretty much the same. are they copying each other?