Because the Celtic, Germanic and Slavic tribes were still, well, tribal and living more or less nomadic lives focused on surviving.
Really? The neolithic revolution had spread through most of Europe before the Bronze Age. These were largely sedentary lifestyles.
If we compare European culture to contemporary Bronze Age cultures of the southern Mediterranean... While Egypt and Sumer were on their first rise and invented writing, Europeans built megalithic structures and large tombs, which also requires a certain level of societal structure and collaboration of larger groups. They may have been less centralized and hierarchical than Egypt, but that's not necessarily a bad thing.
Trade routes covering continents are attested even in the Neolithic age. Goods and technologies were exchanged between different regions and cultures. A wide spread of ceramic types and burial customs shows that people connected across wider distances. Oetzi demonstrates sophisticated equipment that far exceeds mere survival and "swinging from trees". Stonehenge, New Grange, Goseck and the Nebra disc prove that they knew astronomy.
The Tollense valley battlefield speaks of well organized groups and potentially substantial conflicts.
If we compare Greek to contemporary Celtic culture in the first millennium BCE, the Celts had multiple large hill-top fortresses housing a substantial population. Their chiefs imported ceramics from Greece and mixed those styles with their own culture.
The jewelry, ceramics and textiles of the Celtic tribes in the Iron Age show a high quality. The famous Nordic steel which the Roman legions prized for their armies came from a Celtic region.
Just comparing available technology and the level of arts and crafts, European cultures were a bit different and less centralized, but certainly not "more primitive" than near Eastern or Greek contemporaries.
I see no reason at all as to why they wouldn't have had philosophy.
They were also oral cultures and didn't tend to write much. The Druids, for example, were expressly forbidden from writing anything down about their rites or beliefs. All those other cultures you mentioned were settled, literate civilizations. We don't have much from when the archaic Greeks were still living tribally and transmitting their culture orally, either, if you see what I mean.
This does not mean they didn't have intriguing insights about things (we know that "primitive" tribal peoples usually have complex cosmologies and views about life and the world; they're still human and it's just natural to us to wonder about such topics), but rather their cultures were focused more on other pursuits and they didn't write things down.
I agree that the lack of writing and perhaps even a taboo against writing a things down is a huge point. It's also, in my opinion, the ONLY reason that there is no "Ancient European" philosophy: it simply didn't survive, because it was oral traditions.
Always remember: absence of evidence is no evidence of absence.
And in this case, what we know about European cultures since the stone age, as well as comparison with oral traditions around the globe, "it didn't survive" is far more likely than "they had none"
Oh, and then there's also this massive 19th and 20th century bias of a historical science that used to focus on Greek and Roman written sources and faithfully copied their propaganda against the "barbarians" and took it for fact... Those stereotypes are still being repeated in movies and popular culture.