Why sequence number is random?

Protect TCP from data error.
A succeed to connect to B.
A send “hello” to B but B failed to receive that, while the "hello" message wandering in the network.
A closed the connection after too many times of resending.
A reconnect to B with same socket(with same IP and port) in a very short time.
If the message "hello" then arrive at B, it will be treated as valid, because it's SIN is valid!