Otherwise, if it was that easy on bitcoin minus creating or using side-chains, we would be having a ton of tokens on the Bitcoin network that are not just NFTs given that Bitcoin is the oldest network.
We do have them. We have Omni (even with some USDT tokens there) and Counterparty. And they don't need to run natively on the chain. Most NFT arts on ETH aren't even stored entirely on the blockchain (but linked to a .jpg on the outside) and they are doing just fine.
Ethereum is turing complete, has a higher network capacities, most devs and token communities, so they won the "tokens" race. But that doesn't mean it's not possible for us to do something similar on the Bitcoin network if we wanted to. You're free to create your own token through Counterparty today:
https://counterparty.io/projectsIt's not if we should, if it makes sense or if it's worth it, but if we can (and yes, we can).