That is one terrific question! Here are two brief ways to understand why Shavuot is only one day.
One explanation, offered by Nachmanides, is that Shavuot is in a sense the concluding (i.e., eighth) day of Passover. All of the days between Passover and Shavuot are akin to the Intermediate Days (Chol HaMo’ed) of a Festival. What does this mean? Passover, which represents physical freedom, is not alone sufficient. It must be coupled with the spiritual goals and ideals represented by the giving of the Torah on Shavuot.
An idea from the Maharal of Prague treats us to a deeper understanding. He writes that the number seven represents the physical world, with a natural cycle of physicality and spirituality combined. The number eight, on the other hand, represents pure spirituality. Seven is the number of the days of Passover, the time of physical freedom from Egypt, but the true culmination of that Exodus was only when the Jewish People stood at Sinai and received the Torah from God. In a sense this means that Shavuot is not really just one day. Rather, it is like the “eighth day” of the “Passover-Shavuot Festival”. Shavuot is the one day that celebrates the completion of the process of our receiving true freedom, a process that only began at Passover.
There is a second reason I’ve heard for Shavuot being “only” one day that I also find quite fascinating. We are supposed to view each and every day as if the Torah is being given today anew. Therefore, celebrating Shavuot for one day signifies that every single day is to be appreciated by us as if it is the day of the giving of the Torah.