Paper Weights

Why is paper so oddly measured and weighed? I mean everything is the same as everything, yanno? Though why can't paper be the exception. Sure, it would be interesting if every human could speak one universal language. Or a universal currency, universal love even! But none are as necessary as paper weight is to me right now. So here is a little easy to read comparison on a commercial level for my own benefit. 

Trying to make postcards. Im actually going to make a spreadsheet on just save it on my gdrive. Much easier!