I'm just wondering whether it's practically not possible? Because on EtherScan, all my tokens are automatically listed, and when I choose one, I see the contract address and the number of decimals. Why do I have to add these manually on MyEtherWallet?
MyEtherWallet is a simple wallet, it allows checking balance and sending new transaction. But it doesn't track old transactions. It is a simple service to maintain. Everyone can deploy their own instance!
Etherscan is a block explorer, it traces all transactions in mined blocks. It is a much more complex service, for example it needs several databases to store processed transactions. Developing and maintenance are more costly accordingly.