you just had really really bad luck on your build. i've owned pre-built pc's only twice; when i first started learning with my dad's pc back in 1998 (Cyrix DX33). i disassembled that and reassembled everything and did not boot, went to the store and they fixed it as i watched and learned that you need to listen to beeps to know which component is not connected properly. sidenote: a classmate of mine upgraded his system so i bought his old pentium 200mx with board and ram and that kickstarted my custom pc building.
and my current pc; we just migrated and the laptop i brought with me is starting to show signs of age so i opted to go pre-built because it is cheaper. its a M92p SFF with an i3770 that i have since upgraded the ram, installed a 1050LP, removed the 250gb hdd and dvd drive and replace them with a 500gb ssd and 4tb hdd. will be picking up and installing my pcie x1 extender and pcie x1 wifi/bluetooth later.
everything in between those are all custom pcs that i've built from scratch or upgraded piece by piece with whatever money i could save. i enjoy tinkering and troubleshooting so DIY really suits me.