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/
Those sites are like Revoke.Cash site or different? If like Revoke.Cash are they cheaper gas fees than revoke.cash?
So after using established de-fi sites like Uniswap is it good practice to revoke straight away after using uniswap? Regardless how reputable and established a de-fi site like uniswap is, it is instead the smart contract that I should be worried about not the site uniswap because uniswap does not write the smart contracts?
To confirm there's no way to revoke for free, gas fees must be paid for revoking? If so De-Fi is expensive to stay safe and secure using it.
I know what approval amounts are in smart contracts but what are revoke signatures in revoke.cash?
Does it cost gas fee to edit the existing approval amount?