> Why?

Because Google requires it. Google is the only major ad network that requires that you use their own ad serving product (GAM) in order to have full access to their inventory.

So your options are to use GAM and have Google inventory plus those of all other companies, or use any other ad server and have access to inventory of all companies except Google. The latter is essentially financial suicide since Google controls more than 50% of publisher demand.

It seems you are looking at it from the perspective of an advertiser?

Why would it be financial suicide to only advertise on non-google inventory? Isn't that like saying advertising only on certain media and not on other media (say only newspapers but not billboards) is suicide?

I am sure there are companies out there who never advertised on Google property but did not die?

No, I’m discussing from the point of view of the publisher.

It’s financial suicide to lose 50% or more of your revenue because you don’t want to use GAM to manage your ad inventory.

It seems most small publishers just put the Adsense code on their website. Is that equivalent with using GAM?

Just putting the Adsense code on a website is what a hobbyist who's making a few bucks/month would do, not someone who is running a business or making a living from their service.

It makes sense that the case is not focused on this group, the extremely low-end/hobbyist range of ads is not really the same ecosystem and is not remotely as important.

Really? I know people making a living by running one or multiple sites with adsense on it. What would you tell them they should do instead?

Two approaches:

Reseller approach:

Partner with a reseller that gives the publisher access to the reseller's own AdX and header bidding with a 15% or lower cut. Preferably a Google certified partner. A good reseller will work with the publisher to continually optimize/improve.

Independent approach:

Sign up for GAM, setup AdSense within GAM (via line items) so that other ad units can compete via dynamic allocation.

Setup header bidding (prebid.js) on their site. Contact all of the companies (Appnexus, Index Exchange, etc) and try to get partnerships. Requires technical competence, but they can hire someone to do this part if needed.

Header bidding will compete in real-time with AdSense and boost revenue. You can also supplement it with other non-display units (video, floating ad units, etc) from other companies specialized in those areas.

Once traffic is high enough, they should be invited to join AdX instead of AdSense.

Would strongly recommend beginning with the reseller approach, as there a lot of things you need to do properly to ensure you're not shooting yourself in the foot, and they can help you learn.

A good resource is the /r/adops subreddit.

Nice, thank you! I will talk to my "living on ad revenue" friends about it and see what they say.

I am surprised that all of this isn't automated and baked into Adsense already.