I always build. Mainly because I feel that I can have a MUCH higher quality PC doing so. The biggest benefit is the quality of parts, usually the prebuilds use VERY cheap parts despite the specs looking good. Sure, they use a 9700K CPU however it may be coupled with 2400Mhz ram on a green PCB with really loose timings. A lowend cooler, resulting in temps of 80C temps during gaming, a GPU with a terrible heatsink as well and a power supply that isn't even rated bronze and is just waiting to blow. Prebuilds usually use crappy motherboards as well. They've got to make their money somewhere. When I build my own I know that I can ensure it is cooled right, fast and built with quality components.
Yep, this is basically my reasonings... Aside from that, i just like tinkering, and enjoy upgrading, building PCs anyway..
With pre-built there will always be shortcuts, as above, whether PSU, or something else...
I like to hand select my own components... and i guess have my favourite manufacturers, giga for mobos, eVga vid cards, seasonic psu, etc...im due to update, always went intel
but that ryzen 3800x looks great value.