DISCLAIMER: I have no desire to get rid of the Grundgesetz/act on article 146, as it's a mostly fine law, as far as constitutions go, and I also think Germany is clearly a sovereign state, not a puppet or serf to the U.S. or whatever. But a growing subculture here seem to buy into that stuff, which worries me, mostly because I'm scared to find out what bothers them so much about the Grundgesetz that they would rather overthrow it in its entirety than continually reform it. I think Article 146 is a neat little quirk.
Article 146 states that the Grundgesetz becomes void, if and when the reunified Germany freely chooses a constitution for itself. Germany has been reunified, and obviously a new constitution has not been established, so clearly the Grundgesetz is still binding.
However, as far as I know, the term "Grundgesetz" was chosen explicitly because it was intended as a temporary law, a "true" constitution would only be enacted if and when the GDR states were to reunite with West Germany, such that all Germans could be represented in the process of establishing a "real" post-war constitution. Hence the existence of Article 146.
Wikipedia flatly states that, at the moment of reunification, the Grundgesetz became the constitution for all of Germany. Clearly, this is legally true, but the change from using the word "Grundgesetz" to the word "Verfassung" suggests that the planned inclusion of the eastern states (and the German people in general, since the Grundgesetz did come into existence at the - quite reasonable - behest and under direction of the Allies) in the writing of the constitution had taken place, which, to date, it has not. (EDIT: Though I guess one could argue that the willful integration of the GDR into the Federal Republic implies consent to the Grundgesetz, meaning article 146 has de facto run its course.)
Why not? Or perhaps rather: Why would the Grundgesetz ever have been intended as a temporary placeholder? It's obviously much more efficient to keep going with an established constitution, so why even put that language in? It's not like Germany didn't have some outside imposition coming to them after the war.
I have acquiantances who believe that this failure to trigger article 146 and write a constitution "ourselves" (whatever that would even mean in practical terms) is a sign of continuing serfdom of Germany to the Allies. I think this is, at best, an overly dramatic take on what's actually the case, but they have at least gotten me curious to learn why the Grundgesetz is the way it is. It's too murky a subject for me to figure out a good answer; I tried.
So, why has Germany never acted in either direction on Article 146?
Because the West German founders failed to anticipate East Germany's collapse.
The basic idea of the Grundgesetz was always to reunite Germany at some point in the future. As you correctly pointed out, this is why the word "Verfassung" (constitution) was avoided in favor of the less binding and less grandstanding "Grundgesetz" (basic law).
However, the idea that was behind writing the 146th was that both the Federal Republic as well as the German Democratic Republic would stop existing as soon as 146 was triggered; or more precisely that 146 gave the West Germans leeway to ignore the basic law when drafting a new constitution in cooperation with East Germany. The writers of the Grundgesetz did not anticipate that East Germany would collapse in on itself; they instead expected long and hard negotiations to at some point find common ground to stand on and a new constitution to be written.
What instead happened was that the GDR more and more stumbled towards collapse in the late 1980s, whereas the FRG generally kept prospering, as it had since at least the mid 1950s. The GDR's political system, corrupt and inefficient, was completely delegitimized in the eyes of East German citizens and the East German communist government did not wield the authority in either country's population's eyes to negotiate on the same level as the West German administration.
This, in effect, meant that, unlike what the founders had anticipated, there was no negotiation from man to man about abolishing both states and forming a completely new one - instead East Germany was negotiating to join West Germany. So then, we suddenly have a West Germany that contains East Germany as well, with East Germany operating under West German - now united German - law. And now the united Germans seriously ask themselves: Do we even have to throw out the Grundgesetz?
To which the answer, as you also correctly pointed out, was rightfully no. The German basic law is perfectly fine as is - it fortifies a liberal democratic nation state that accomplishes to provide individual freedoms and economic prosperity to its many citizens. The one thing that one could change to accomodate the founders would be to abolish the basic law and then immediately re-adopt it as a constitution and without the 146th.
But, really... why bother?
So, in essence: The reason why the West German basic law was framed as replacable and temporary was to enable reunification through the drafting of a new, shiny all-German constitution on equal footing with both German governments. The East German government collapsed before that could happen and was forced to accept what amounted to annexation into West Germany without the proceedings that the founders had anticipated to be inevitable.