I have no idea why that would be true? I assume pretty much anyone with enough money for an internet connection has some value to advertisers.

You still get advertisements, but with DNT they wouldn't be targeted to your profile.

I'm not sure, but let's assume they are. Doesn't respecting do not track reduce their value even further? I'm not arguing that ad companies shouldn't respect so not track, just considering the incentives and disincentives in front of them.

The main motivation is to have some standing ground to prevent legislation.