Is it ethical that the PhD student is not offered a chance to become the communicating/ corresponding author? Usually a supervisor has to focus on many project at once but the PhD student on the other hand has only his project to focus on, PhD student is the person who usually does all the literature work as well as the experiments, so technically he is the master of that particular project then why the PhD student is never offered a chance to become the communication/corresponding author of those manuscript that are published by him on his work. One could argue that in the first 2 years of the PhD work he might have less command over the subject to which I might agree but what about in the last stage of the PhD program or after the PhD program for that matter. Why is he never offered a chance to become a communicating/ corresponding author?

Communicating/ corresponding author here represents the person who would respond to queries and reply to emails, where as the last author is usually the supervisor who wrote the project or took the lead.
Things might vary across fields, but I think that you might be overestimating the impact of being the corresponding author.

In my experience the corresponding author handles the communication with the journal (hands in the article and revisions, copyright form, etc). Furthermore, you might get a question every once in a while from a fellow researcher, but they are mostly like: "can you send me the article", or "may I use your figure".

Reasons for your PI to insist on being the corresponding author might be that articles may get questions long after you left the institution, and in some fields the corresponding author indicates who is the PI.

I would not worry too much about this. When your publication list is assessed, people will look how often you are the first author (when that is applicable for your field, some fields put the authors in alphabetical order). And when somebody has a question about the content of the article (which happens less frequently than you might expect) they either write you directly, or your PI might forward you the question.