Why so much fungibility creeping in?

I’m developing smart contracts for ERC20 and ERC721, and I’m going with the proxy pattern, so the contracts can be upgraded(if required) and because it seems like a good idea for someone learning how to work with the block chain.

While I’ve been learning/working on these contracts I discovered the ERC1155 tokens, which are a combination of both, but rely heavily on off chain services to provide the details that aren’t on chain.

A quote from one of the guides I read about a benefit of the 1155 token is:

“ you’ll notice that the item’s information is included in the metadata, but that information isn’t on-chain! So a game developer could change the underlying metadata, changing the rules of the game! “

I think I’m still enamoured with the romance of an immutable contract, but all the latest contracts/proposals I’ve been looking at are providing more methods to allow changes and extensions long after deployment.

Perhaps I don’t understand the 1155 specification, or mutable contracts, but what value do they add to the idea that nothing can be tampered with? I get the savings in gas for things that deal with game like content, but don’t really see the utility of storing transactions on chain if the content for the token is going to change.

Why build a dapp at all, when you could just make an open source project on GitHub and achieve a similar result(less the gas). Instead of hiding transactions behind iam policies, just publish them on your url/transactions?

So I think there are a few questions here, but what I’m basically asking is why is everything migrating away from immutability?

(I’m a new developer, please forgive my ignorance)