why is it by default for every transaction that the approved limit is set to Unlimited/Max and to revoke this it cost gas fees 
When you are interact with smart contracts, it's on chain activities and need to pay on chain transaction fee. More details from a guide of Coinmarketcap can help you more.
How to Revoke Token Approval.
You can try with two other sites for smart contract revoke.
https://etherscan.io/tokenapprovalchecker
https://app.unrekt.net/