It's quite easy to get a measurement on your screen. That doesn't necessarily translate to measuring the impact of ad spending.
If I have social media ads, paid search, and retargeting ads all live on multiple platforms, there is undoubtedly interaction among them and the quite-easy self-serve ad tool is likely to show me a simplistic "last click attribution" at best.
Often on Google I see ads right above a organic link to the same site. I wonder if marketing teams count the ad click as a "win" when in fact I was looking for the company because I learned of it via word of mouth.
Most analytics platforms will report on any straight direct line between spend and incremental revenue. The hard part is measuring the indirect part, as you mentioned, but it is possible.
It's easy to bullshit yourself into thinking your ads are much more effective than they really are. It's even easier to bullshit the people who pay you to do ads for them. Advertising and analytics platform happily oblige both use cases.
As 'sokoloff wrote, it's easy to get a measurement on the screen. But gaining real insight from it, or even knowing whether that measurement makes sense, requires a certain amount of analytical thinking and knowledge of statistics. Maybe among corporate marketing departments such skills are plenty, but in my experience, it's not usually the case with small marketing agencies serving smaller companies. Bullshitting has a better effort/reward ratio, particularly when your customers understand statistics even less than you.
If I have social media ads, paid search, and retargeting ads all live on multiple platforms, there is undoubtedly interaction among them and the quite-easy self-serve ad tool is likely to show me a simplistic "last click attribution" at best.