I think maybe we are just talking about different protocol layers? It's a bit like asking why IEEE 802.3, the Ethernet standard, doesn't address security. Security is built on top of Ethernet. At least, that's the typical way of implementing security.

The other point is that a telecom does need to implement security measures, but this is primarily to protect itself. For example, their system management messaging, routing tables and routing protocols, have to be secure, to prevent a hacker from disrupting the system. But it doesn't do much good for the telecom to secure user traffic, if ultimately, at the edges, the traffic has to be sent in the clear. If a user of the telecom wants security, that user must install a solution that protects traffic end to end, from inside his PC or smartphone, all the way to inside the PC or smartphone at the other end. Or in the case of an enterprise network, security could be implemented until the link is inside that enterprise's "secure enclave," perhaps not all the way to the end system. So for example, the enterprise may deploy secure tunnels, through a telecom network, where the end points of those secure tunnels are inside secure enclaves.

Otherwise, anyone along that path would be able to monitor the communications. And there is no reason for a user to trust the telecom itself, ultimately, if that user needs security.

Security can and is implemented at any protocol layer. From the standpoint of a user, though, any security measure that is not end to end would be of questionable value. That's why people use Transport Layer Security so often, when dealing with communications across telecom or other network services. It's security built over the telecom link, carried transparently by the telecom or by the digital network.

I think maybe we are just talking about different protocol layers? It's a bit like asking why IEEE 802.3, the Ethernet standard, doesn't address security. Security is built on top of Ethernet. At least, that's the typical way of implementing security.

The other point is that a telecom does need to implement security measures, but this is primarily to protect itself. For example, their system management messaging, routing tables and routing protocols, have to be secure, to prevent a hacker from disrupting the system. But it doesn't do much good for the telecom to secure user traffic, if ultimately, at the edges, the traffic has to be sent in the clear. If a user of the telecom wants security, that user must install a solution that protects traffic end to end, from inside his PC or smartphone, all the way to inside the PC or smartphone at the other end. Or in the case of an enterprise network, security could be implemented until the link is inside that enterprise's "secure enclave," perhaps not all the way to the end system. So for example, the enterprise may deploy secure tunnels, through a telecom network, where the end points of those secure tunnels are inside secure enclaves.

Otherwise, anyone along that path would be able to monitor the communications. And there is no reason for a user to trust the telecom itself, ultimately, if that user needs security.

Security can and is implemented at any protocol layer. From the standpoint of a user, though, any security measure that is not end to end would be of questionable value. That's why people use Transport Layer Security so often, when dealing with communications across telecom or other network services. It's security built over the telecom link, carried transparently by the telecom or by the digital network.

I think Albert has well answered your question. Of course telecomm engineers are heavily responsible for ensuring continuity and reliability of the provided services, which they call security of service. However, I think you are concentrating on privacy and hacking security. 

Security is a big concern in telecom sector and a lot of work in progress in this area from the last few years.

To support Khan' viewpoint, here is a reading list.

https://scholar.google.com/scholar?hl=en&q=telecommunications+security

thank you very much everyone for your helpful answer !

i agree with khan's point of view about telecom sector sensitivity in any country , especially in mobile communication and transmissions .

everything in telecom turns to be digitalized as this makes this sector greater than it was , it also has cons which makes it even more sensitive to a lot types of attacks ! ,such as ddos attacks ,third party ,even social engineering is a big threat for security nowadays