I don't understand why we have to put everything on chain?

You don’t have to. But you should let others do it, or else you’re restricting growth. That’s central planning and very damaging.

I mean i can put my post on twetch or weiblock without on chain, but I can still earn some funds by providing good content.

Not everything, just things you want to be able to share in a secure / reliable way outside of today's main data-sharing systems, or that you want work in a distributed manner that lends itself to blockchain.

I know its a bit of a cliché but when someone offers you a free service, you are not their client but their product for sale. They sell your data to their actual clients who want to advertise their products to you.