I can't really find a straightforward answer to this.
I read from Investopedia that NAV of open-end funds is calculated at the end of the day. So I believe that's the reason for why the fund can only be traded when the market closes. However, I'm not sure if this is the actual reason behind why one can only trade shares of the open-end fund once market closes.
Because shouldn't it be possible for NAV to be calculated instantaneously during the market hours (perhaps with computers/programs compiling real-time prices of all the stocks held by the fund)? If NAV can be calculated in real-time, then that'd be much more advanced than only calculating NAV at the of the day (we're in 21st century after all)? Thereby, this real-time NAV would allow open-end funds to be traded during market hours?