We completely understand your confusion on this! Unfortunately, the airline industry generally makes it more expensive to make changes to your itinerary rather than purchasing new flights outright.
When you purchase an economy class ticket, you are essentially buying a class of service in a certain price bracket, which is what determines how much your ticket originally costs. This is why you will see economy tickets available at many different prices for the same flight and seats. The class of service also determines how restrictive the fare rules are on the ticket.
When you want to make changes to your ticket, the airline requires that we price the change in the same or a higher class of service (price bracket) as the original booking. As the calendar gets closer to the date of the flight, the classes of service on the flight become uniformly more expensive. This means that you may see a better price offered in the app for a flight, as it will be in a lower class of service than we are allowed to change your ticket to.