Is about security reason.

Because you want to change "Email/Telp" to make sure you are the original person who owns the email/number. What happen, If you email got hacked but there has some security withdrawal with a number and they can process just by using the email.

You can lost the asset, any change about the information should by KYC. Is already basic process, even on some exchange can do that or could be the worst you can't change any email/number.