My current X99 system was built in Fall 2014...
But this is only because finances haven't been where I've needed them to be for a long while, and systems tend to last a lot longer these days anyways. Before that I was building new systems for myself every 4-5 years. I suddenly got a free X470 system with a 2700X a few months back that I've already prepared that I'm about to switch to. Hoping to throw 5700X3D or 5800X3D in there come Black Friday/Cyber Monday. Of course this sh1t is old too, but still an upgrade over X99/Haswell-E. I always keep current-gen plans for building new systems though, and my eye had been on Zen 5 for building an all-new, actually-planned system. I ain't down with Intel's big.LITTLE P-core/E-core nonsense.
I'm no stranger to keeping old sh1t. My car is 25 years old, and I've been driving it nearly that whole time...
I blame consoles for that. Ever since 7th-gen, console development has led game development on the whole. With the exception of a scant few games, most games are primarily designed for consoles first, and PC is an afterthought. The only thing that has really helped on the PC side is consoles adopting AMD64/x86-64 architecture. By essentially becoming PCs, this also helps make the ports to PC slightly less worse, and I'm pretty sure that the only reason some ports to PC even happen these days is because they're already most of the way there with the console versions. But yeah consoles haven't gone past 8 cores in a couple generations now. The lowest common denominator holds back everything for everyone. I mean we had AMD64 since 2003 and games didn't switch to 64-bit en masse until 8th-gen consoles in 2013. D3D11/OpenGL4.5-class graphics also didn't really take off until 8th-gen consoles some years later. Consoles had a big part in the development and usage of D3D12. Vulkan (and in a way D3D12) is based on Mantle, which AMD designed for consoles first. Game development tends to work out better when you develop a game for PC first then port down to consoles, not the other way around. At least some devs still understand that. No very many, but if you squint hard enough, you can see a few. Sorry for the old-man rant. I'm actually not a console hater. I've had at least one console every generation since 2nd-gen (and I still have every single one except the Atari 2600 which I suspect was stolen), although I think this current 9th generation might be the first time I don't get any of the consoles.