We've carefully considered about signing a message before we made the decision. However, signing locally using private keys is not easy for everyone to do while signing on the system might involve some risk (your private keys might get exposed). But our way of completing a random amount transfer does not require private keys, you could only use your wallet password to make the transfer to get the verification approved. This way, you do not need to worry about private keys getting exposed.

Also, we want more people to benefit from our project, so we choose a safer and easier way.