In games where you have to use an account to sign in or otherwise authenticate yourself (e.g MMO games), it is often prohibited to share your account with other people.
[...] You may not share the Account with anyone, except that if you are a parent or guardian, you may permit one (1) minor child to use the Account when not in use by you. You are liable for all uses of the Account that has been enabled by you […]
[...] You can’t share your account or Login Credentials with anyone. You can’t sell, transfer or allow any other person to access your account or Login Credentials, or offer to do so. You’re entirely responsible for maintaining the confidentiality of your Login Credentials. [...]
What is the reason for this? Is it because of legal reasons and / or security reasons? I imagine account sharing would not be a problem if there weren't some real-world consequences for the company, not just the end-user.