And actually, there are situations where the 570 could use over 4GB, so the 8GB can make sense. Especially if i want to run high resolution textures. Something that a 3GB 2060 will not ne able to do, and hilariously may even perform less than the 570 in that situation.
Please note that I did
not say there is no use cases for having more memory, but just because
some may need it, doesn't mean everyone needs it. For many, the cheaper cards offer much more value. It's no accident that both AMD and Nvidia offer their RX 480/580 and GTX 1060 in low and high memory configurations respectively.
And as I've mentioned, the fact that a game allocates more memory doesn't mean it
needs it. To evaluate that, you need to evaluate reductions in performance and/or rendering quality.
The fact is, AMD is offering more value in the mid-range. And 570 isn't low-end, it's lower-mid-range.

But feel free to defend getting less hardware for more money. Seems basically the whole idea of Intel / Nvidia these days.
If you keep expanding it, it's no longer the
mid-range.

In the mid-range, GTX 1060 3GB/6GB have been the better choice over RX 480/580 4GB/8GB. The only slice in there where AMD have no direct competition from Nvidia is the new RX 590, but the only argument for this one is if you can't afford GTX 1070. RX 590 is still a little odd, considering how close to GTX 1060 and RX 580 it really is. In general, AMD is hardly competitive in the mid-range, and except for possibly the RX 590, they can't claim to offer better value. This is only going to get more challenging as RTX 2060 arrives.
In the low-end, AMD have more compelling options, like the RX 570 vs. GTX 1050 Ti.