i was speaking in software parlance.. but the idea is to segregate parts by explicit functions, of course the screw holding the graphics card in place is also performing a function, a motherboards job is to connect other parts to the cpu, there are no explicit functions a motherboard performs (other than maybe onboard sound processing and ethernet)
if what you say about overclocking is true then it only strengthens my original argument about x670 pricing
By that logic you can consider a motherboard to be something along the lines of a translation layer, API, or dependency - all of which are either necessary, or in the case of APIs, add functionality which have value.
Also, do you want a PC with no sound output or networking capability? (yes I know you can buy them as separate add-in cards, or use sound via HDMI/DP)
Agreee on motherboard pricing with the latest generation, although maybe more like 3x than 5 (my x370 Taichi cost around £200-220, and part of the reason I wanted that model was the white aestetic, so compare to the carrera at £600+)
Then again, AMD motherboards were always cheap, until Ryzen came along and made them the more compelling option for a system.... Intel boards were expensive since some time around Sandy Bridge - although my Abit IP35 Pro XE was something like £86 and was a banger of a board - Heatpiped collers and a debug LED, both features which I really appreciate and you're now in to £500 territory if you want them
Circling back to GPUs, it's basically just market forces and what people are willing to pay, between the ever increasing complexity + a big jump due to covid times (more focus on in home entertainment vs going out, plus reduced costs due to WFH)