Not people's perceptions of said cultures. Africa is over 62% Christian as it stands today, and yet the Wakandans of hollywood practice ancestor worship (representing ~2% of Africa). There's irony wherever you look. We can't seem to move past the past when it comes to minorities.
A lot of christians, maybe most globally, also practice veneration of the dead. Sometimes syncretically, but also from an external anthropological viewpoint the communion of saints is indistinguishable from ancestor worship. The christian theological understanding of it is very different, which is probably why we don't use this term for it. But's it's there.
I mean, (the fictional nation of) Wakanda is depicted as a very isolationist country, and it's reasonable to expect that Christianity wouldn't have taken root if foreign missionaries were expelled throughout its history. Tokugawa-era Japan may be the closest comparison, where its isolationist policies resulted in fairly high retention of indigenous religions even through to modern times. Today less than 2% of Japan's population is Christian, whereas nearly 30% of South Koreans are Christian.