It is reasonable to believe that Starkiller Base is built this way because its design is shaped by the First Order's existing beliefs and because the First Order's plans have no need for it once its intended purpose is fulfilled.
The First Order, founded by the majority of the Imperial military leadership that survived Jakku, inherited the Imperial military and political doctrine its founders are still loyal to. In Legends, we call it the Tarkin Doctrine. In canon, it does not have a name, but is still the same philosophy that led to the construction of the Death Stars: the Imperial military are enforcers of the New Order. In allocating resources to this purpose, both the Emperor and Grand Moff Tarkin prefer constructing massive, imposing shows of power and symbols of fear (superweapons) rather than concrete, efficient and effective force (more fleets, newer and better tech etc). This is why, to this day, the heirs of the Empire still prefer ever larger and more imposing symbols of fear and power, however unwieldy.
To understand the First Order's plans, we should first examine what we already know. The Wookiepedia article on the New Republic cites Before the Awakening as the source for the following:
Despite its outward appearance of unity, many members of the Republic Senate were in fact in collusion with the First Order, with senators such as Erudo Ro-Kiintor discretely accepting large sums of credits transferred through the Corporate Sector Authority and third-party corporations. These senators frequently delayed, derailed and dismissed motions regarding sanctions against the First Order and increased support for the Republic Navy.
We can observe the following visible elements of the First Order's plans:
- Subvert the New Republic's warfighting capabilities and political unity
- Destroy the New Republic's central government and fleet with Starkiller's opening shot
- Destroy the Resistance's central command with another shot
- Once war is inevitable after Ep VII, the First Order very likely intend the following: while the New Republic is still reeling from a loss of the central government and the accompanying infrastructure and hierarchy, seize back control of the galaxy while Imperial loyalists defect to the First Order or continue to subvert the New Republic from within.
Basically, if all had gone to plan, there's likely no need for Starkiller Base after that.