In the last decade that I have been running osTicket this comes up fairly often. Agents can reply to tickets via emails, but doing so posts to an Internal Note., and is not a response to the User (client) that opened the ticket. There are posts on github and here where people have figured out how to change this behavior for various different versions dating back to 1.6ST. If you would like this functionality you can always modify the core files yourself and add it. Search should point you in the direction of the other threads where this topic is discussed, and those should point you in the right direction as to how to achieve it.
I do not know if the devs intend to change this behavior in the future (or provide a toggle to change the behavior in the settings).