Because there are few if any good metrics for evaluating the brain’s performance and connecting it to isolated factors, and the experiments are near impossible to replicate. Brain related research is also some of the easiest to churn into clickbait, since it rarely gets into the nitty gritty of molecular interactions.
>Public Significance Statement—The idea that bilinguals outperform monolinguals in cognitive control functions seems to have already been accepted by the popular media and educators, because of a number of influential studies reporting a bilingual advantage. Our thorough meta-analysis, however, suggests that healthy bilingual adults do not have such a cognitive control advantage. The synthesis of 152 studies and 891 comparisons and several moderator variables does not show systematic advantages across the analyzed cognitive domains, tasks, or bilingual population
A quick google search finds several studies about the benefits of multilingualism on the brain.
Here’s a recent one: https://www.fens.org/news-activities/news/speaking-another-l...