In theory, I can look up how to fix anything that goes wrong with my car if I rent the equipment. I can also probably fix someone’s broken leg after a few Youtube videos. But it’s best to let professionals do these things to save time and money from costly mistakes. There are a lot of things that professional media buyers know that you can’t find on the internet easily yet just because you wouldn’t know what to look for. For example, I’ve never met a business owner who knew that you can now retarget only your website visitors who spent 30+ seconds with a specific pricing option centered on the screen. Most only retarget all website visitors, including known bots and those who left without reading anything beyond the header.
But you’re right, if the expense of an ad agency is greater than the profit then that agency won’t be in business for long.