Even then, I think you are being too generous. The stricter criterion is something like: "People who run more ads believe they will personally benefit from running more ads."
Ideally (and occasionally) this means that ad buyer believes the company as a whole will benefit from the advertising. But even if this belief is real, if the influence or income of the decision maker correlates positively with increasing spending on ads (and decreases with cutbacks) it can be difficult to distinguish belief from reality.
All this talk about "belief" like it's religion or something. It's reasonably feasible to get hard, scientific numbers that prove the effectiveness of advertising spend. Ie tracing click throughs vs conversion rate.
"Everyone is doing it, must be good."
I believe this summarizes the ad industry quite well: https://www.youtube.com/watch?v=TysKyHXXtYQ