Primarily because the sources that describe it are so vague/discreet about its composition.

The scholarly consensus, however, (such as it exists) is that the basic ingredients of Greek fire were crude oil, resin, and sulfur. This compound was heated and then pumped through a bronze tube. It is unclear whether it was ignited as it was sprayed or by flaming arrows shot after the mixture had coated its target.

Source: Oxford Dictionary of Byzantium, s.v. "Greek Fire"

Its not that modern writers cannot figure out "greek fire" its that

1. There are several ways and concoctions using contemporary materials to get the effects described in accounts of its uses. We know of some of them, Persians used crude oil, for centuries (known as Naphta).

The Abbasids used Naptha and also had their own version of Greek fire, whose composition **is** known, it was crude oil, with sulphur and resins added. Its method of use was similar to Greek fire

2. The effects described differ in different treasties. Which makes some people suspect that there were different types of Greek fire or the composition changed over the centuries.

*Forbes, R. J. (1959), "Naphtha Goes To War"

*The Age of the ΔΡΟΜΩΝ: The Byzantine Navy ca*. 500–1204*,

It is not that 'we can't figure out' the composition of the Greek/Byzantinian fire, as there are at least several working reconstructions of the substances that are either easily flammable when ignited (Greek fire) or ignite on contact with water (Byzantinian fire) and use ingredients readily available to the Greeks in early Middle Ages. So, from the practical standpoint we very well know how these substances were made. What is unclear is only the fact how well they relate to the actual composition(s) that was used, because of the complete lack of contemporary sources. It is even possible that there were never such sources, because of the high level of secrecy, the recipe for this weapon could have never been written down and was taught personally by those who possessed such knowledge.