You can't overclock
Correct. But I don't ever do that. Also, the Dell already has a CPU much better than in the machine you posted a link to, so think of it as the equivalent of being pre-overclocked.
It has only two ram slots, one PCI-E x16 slot
Correct. But I don't desire more than 16 GB or more than one graphics card.
and no advanced storage functionality.
I don't know what you mean.
It has subpar cooling for a gaming box because it's designed to do light office work.
Incorrect. Also, I have checked with temperature monitors of GPU and CPU during game play. Everything is fine.
It has an ancient hard drive model from Dell mass purchases 5 years ago.
Don't be daft. It has a Seagate Barracuda, 7200 RPM.
Everything in it is cheap and low quality.
Incorrect. Intel motherboard (which I trust much more than others), Intel CPU (which I trust much more than others), and name-brand drives. What is cheap is the power supply, but as you will recall from my 1st post above, I replaced that with a Corsair. Also, as you will recall from my 1st post above, those cheap power supplies are very reliable.
One of the applications we have for Dells is to run our DNA synthesizers. Those computers stay in service for years, running 24/7. At least one (if not more) was nearly 10 years old (and still working fine) before we replaced it. If the computer running the synthesizer crashes during a run, it can cost us up to about $30,000 retail worth of product lost, and we have to resynthesize. Also, we occasionally sell these synthesizers. An instrument costs the customer approx. $300,000, and a Dell runs it. If the Dell on such an instrument fails, it costs us up to thousands of dollars to take care of the problem (if we have to fly people around the world to deal with it).
I would not trust a build-it-yourself system for the above uses.
When you build your own you can choose better components for your needs.
Yes, of course. No one disputes that. It's just that it costs you more.
It has windows 7/8 pro only because
If you don't want the pro OS, you can get the same computer from the "for home" section of Dell instead of the "for work" section. The "for home" version comes with Windows 10 home and more RAM, and the price is $480 instead of the $500 system I got.
I want the pro version of the OS for my home computer, though. I would pay more to get it. I use my computer for more things.
Alienware.
I'm not talking about Alienware. Those don't have the same performance/price as what I'm talking about here. They are irrelevant to what I'm saying.