Why are TCP/IP networks considered unsecured?

It's not as if designers work to build insecurities into protocols or operating systems. It is really more an issue of priorities. TCP/IP was designed with usability in mind.

In example, consider ARP; it is a two-step process that consists of a request and a response. Little thought was given at the time of the development of ARP that someone may actually send unsolicited ARP responses for the purpose of ARP poisoning. Other protocols and applications of TCP/IP also have security issues, such as ICMP, RIP, FTP, SNMP and Telnet.

Protocols like IPSec were not originally envisioned, and it is actually an add-on to IPv4.

For more information, view this tutorial on understanding TCP/IP from FreeSkills.com.