That pre-built is not terribly priced, considering you're getting an R5 1400 and RX 580. However, I can guarantee it uses a very low quality PSU. It's also likely running a cheapo motherboard. Slow RAM, which is a hinderance with Ryzen: you're looking for 2666MHz minimum IMO, ideally 3000/3200MHz.

If you're building a system, I'd be building something like this;
[ Register or Signin to view external links. ] / [ Register or Signin to view external links. ]

CPU: [ Register or Signin to view external links. ] ($94.00 @ Amazon)
Motherboard: [ Register or Signin to view external links. ] ($74.39 @ OutletPC)
Memory: [ Register or Signin to view external links. ] ($88.98 @ Newegg)
Storage: [ Register or Signin to view external links. ] ($54.99 @ Amazon)
Video Card: [ Register or Signin to view external links. ] ($299.99 @ Amazon)
Case: [ Register or Signin to view external links. ] ($24.99 @ SuperBiiz)
Power Supply: [ Register or Signin to view external links. ] ($49.99 @ Amazon)
Total: $687.33

If you're not going to overclock the CPU, I'd recommend going i3 instead of R3, but that would cost an extra $10-15 or so.

The reason that pre-built managed to fit an RX 580 and R5 1400 is simply because so many corners have been cut everywhere else, which really impedes your potential further upgrdes in the future. It also just means that it's pretty low quality overall.

Everything I've included in the parts list above is pretty solid. The case isn't that good, but it's $25 and it'll do the job. For the price, it's great, but it's definitely something that coule be upgraded in the future. Still better than the pre-built case though. Also I haven't included an HDD, just an SSD. 240GB is plenty for your OS, some basic programs, GTA V, and Fortnite with probably some room left over, so it will be fine for now. You will however, likely want to add an HDD at some point in the future, which is really easy, so long as you have a spare $40-50 to buy yourself an HDD.

It is not a huge difference anyone between a custom built PC and a [ Register or Signin to view external links. ] anymore but i do prefer building my own DIY Pc because i have more control over the components.