That's definitely not low-end. More like mid to high-end.
Low-end is still 2 cores, 4 GB of RAM, with integrated graphics.
Yes it's more of a mid tier system but it's a low end system that you would get to play games on.
Also given that games still target the consoles you can get away with 8GB of ram easy.
Further, no one targets 16 GB RAM for a game. Not even the non-released, next-gen consoles have 16 GB dedicated CPU memory.
Similarly, not everyone one has an SSD and therefore companies cannot target that. Only in a few months with the new gen of consoles we will see actual games targeting SSD speeds.
As with the other points though, a low-mid tier gaming PC is something like 4 core/8 thread, 16GB ram, a HDD for bulk storage and an SSD for system & some select game(s). Someone that is buying a Macbook Air or an Ultrabook or something with only 2 cores is selecting it for something else rather than gaming. So irrelevant to the discussion.
The reason no big publisher targets 16GB for a game is because of these old consoles. The games can and do use more memory if available but they are limited by having to target these low spec consoles. They're also required to support the slow HDD speed of the current generation of consoles, so no exploiting SSDs either. This takes significant time and resources to optimise for!
The console AAA games are the ones that target proper GPUs because consoles have them. If they were targeting PC only, you wouldn't see them.
But it is also very much a known quantity and games should be designed to take that into account. Arguably it should be easier to write games that hit 60fps today than it was 6 years ago.
But games have become way more complex in that time, and the expectations of players have grown considerably. So you end up having to put more content in, more features and more data. The disk and memory become a bottleneck and you need to write and optimise some pretty complex systems to be able to handle that well.
PC Hardware has actually advanced since that time as well, so the artists have now more power to create better looking and feeling things in their tools, which means more data to process and get into the game.
Also designers will often surprise you as a programmer with the features and creative solutions to problems. They don't really think and design the same way a programmer would, and don't really design for the system hardware (except for the input/output devices of it).
For a low end PC system we are talking about having 8 cores, 16GB of ram, a decent video card, and an SSD usually or at least a disk capable of 100+ MB/s. Also the platform is pretty easy to develop for and release on.
For a console you get 5 or 6 cores to use, each of which is not as good as a modern CPU core, 8GB of shared ram, and a disk where you have to handle being only able to read from it at 30MB/s. You also have to deal with the black box of vendor tools, that don't necessarily work easily.
So the whole team ends up spending a lot of time actually optimising the game to actually run on these underpowered and lowest spec'd consoles. Removing extra details, downgrading graphics, culling data, compressing, managing memory, just so it can run at 30fps.