When the first cookie law came into action companies sprung up claiming to provide a solution. Rather than actually doing anything useful they'd give you a JavaScript snippet to just dump onto your page and then they'd deal with everything. They'd scan your site and compile a list of cookies and their purpose and present that to your users. It sort of worked, expect the service would frequently fail to recognize certain cookies or part of you site might be behind a login. But it was something you could easily implement and push the responsibility to a third-party for a small fee.
Then came GDPR and these retarded cookie banner companies decided to offer that as a service as well... Basically the same thing right? Well for many of the sites it is, because their goal is find a way to do nothing, or as little as possible, they don't want things to change and here's someone offering them just that.
I hate that it when people are blaming the EU for the nightmare that is consent popups. They aren't required, unless you doing stupid shit. Companies love presenting this as: The EU is making us do this. NO, you want to track people online and the EU is simply asking for you to declare that.
It's truly amazing that companies don't see to problem telling people that they care about their privacy, yet presents them with a list of 600 "partners" whom which they share our data.
So yes, it's laziness, these sites don't want to chance the way they deal with advertisers, because that would be slightly harder. It's also partly incompetence, there's an entire generation of ad people who don't know the first thing about advertising, they know Google Adwords and Facebook Ads.