Probably a weird Question

Hello there

I’m just wondering why people send a greed message to other like hi / how are u..etc

But when people reply them , they never get reply.

I’m not for the blame or rant , just simply wanting to know what is the reason .

Like a lot of guy sent me a hello or how are you,
After I reply and want keep the conversation, they are disappeared.

If you are kind of this persons , could you let me know what wrong with me, or what is you think about when people reply you but you are not reply again, lose interest? Find someone?