There's the fact that soda is packed with sugar and people getting addicted to the dopamine hits. [1][2]

[1]https://wexnermedical.osu.edu/blog/addicted-to-soda [1]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6488513/

In the case of diet soda, I assume that since most people already start drinking regular soda, there's a placebo effect.