For big organizations that have the capacity and data, online advertising becomes a ROI optimization game, and one that they perform quite well at.

For a random business that wants to advertise online, without the infrastructure and data capability to back it, they will struggle to compete unless they exist in a segment full of similar peers. When the former happens, we see articles about how PPC doesn't actually work, etc.

Reality is that it takes engineering work and infrastructure, coupled with some data capabilities to unlock real value in the online advertising space.

As noted, online advertising brought all sorts of insight and visibility over traditional 'offline' marketing channels, but with that comes more savvy competitors that will do all the data things you're not.

Many platforms, such as Google, make A/B testing attribution (ie. incrementality measurements) really easy to perform.

Most people don’t know how to run or interpret A/B tests, or even understand what the confidence levels should be for their sample size.

By and large, people don’t understand statistics (I mostly include myself in that).