Having been at smaller companies without the data, tooling, discipline, and resourcing to conduct viable experiments, and then being at a company that is actually one of the best in the world at it and building solutions for these problems at scale for marketing teams, so many problems come back to the human element in how data is tracked, how teams collaborate or fail to which can lead to pollution and dirty data, and how decisions are made as the moment trade-offs need to be considered it becomes personal and enters messy human relationship territory.

But ultimately from a pure "did this work or not" standpoint you are right. Incrementality experiments are the gold standard.

I think the key point there is "viable experiments", most digital marketing people I've encountered are not into things like deseasoning, controlling, heck, you say "regression" and they blank. Not accounting for trends and covariates would give a misleading impressing even with high quality data.