There are reasons for why we can assume that the west is the least corrupt area of the whole world.

Perception is a proxy for reality, given that you can’t measure the latter.

But for the average person, perception does influence how likely they are themselves to take part in corruption, and that in turn does influence the total amount of corruption in a country. Actual corruption is very hard to measure, so perception of corruption is a reasonable thing to research since it is in many ways "the best you can do" to get some measure of actual corruption.