BGG has a lot of problems with its ranking system, but one of those is that it favours complex games.

The reason being is that complex games are played by fewer people and those who do master it are more likely to give high ratings. Whereas, a less complex game gets played more and is subjected to harsher ratings.

Somebody made a great data analysis of reranking BGG ratings by complexity for the real top games list: https://dvatvani.com/blog/bgg-analysis-part-2