I think you are severely overestimating the protection that can be afforded by the relatively exposed bunkers positioned in those islands. In the age of long-range cruise and ballistic missiles and bunker busting smart bombs, such positions are sitting ducks, especially since the islands in the Spratlys are far too small and flat to offer much in the way of good defensive geography. They're also very exposed, as illustrated by how easily facilities can be ID's by even civilian satellite photography.
The support infrastructure such as the port, runways, fisherman shelters and monitoring stations offer much greater returns in securing our territorial claims than hardened military bunkers.
Also your statement about constructing more robust buildings being more economical because of the the difficulty of transporting materials to the island doesn't make sense. It is exactly that logistical issue that keeps us from being able to build them in the first place. That's why the Coast Guard built their monitoring station using prefabricated parts. Heavier structures may not require as many repairs or replacement as lighter ones, but the fundamental problem remains.
Also, direct military actions in the Spartly's have not been the case, as you so state. No one has evicted anyone else's facilities by force, despite tensions and militarization of islands. In fact relations with other claimant Vietnam are much more amicable, while Taiwan has only kept to the status quo. The only exception to this is the PRC, but even then they have not conducted direct military action to evict other claimants facilities.
In any case this is all a matter of budget and priorities. The claimants that built extensive military facilities were frontline Cold War states that were either military dictatorships (Taiwan) or communist states (Vietnam, PRC) which had had direct experience with open conflict with one another over territories. (See: Battle of Guningtou, Johnson South Reef Skirmish) Such investments weren't made out of pure strategic foresight, but by their experiences elsewhere, which made committing resources to such endeavors much bigger priorities for their governments than otherwise.