Why there is a statement of FIN packet is easy to discard in the networking? In Nmap port scan there is a Covert scanning which use `FIN` for scanning, but it is not accurate, because the `FIN` packet easy be discard in transmission. Why there is a statement of FIN packet is easy to discard in the networking? Is it because of firewall or other IDS/IPS ?

A stateful packet filter will discard any FIN which don't belong to an existing connection, which means that these don't reach the intended target. Stateful packet filters are commonly implemented in firewall appliances and also implicitly (for doing NAT) in typical SoHo routers. Often they also implemented in the target system, i.e. a typical iptables setup on a Linux server.