These are usually business class systems that are usually on par with higher end consumer hardware in terms of reliability if not actually better.
i don't know where you've been finding old OEM Dell systems,
but none of the basic home\office\school models have any of the aspects you have toted.
neither have any of the systems listed in this thread.
what you would be referring to would be business class or high grade science-industry standard machines with a heftier price tag,
not basic store models like what the OP is planning on building a gaming system out of.
but obviously work well enough to dominate the PC industry
they "dominate" the industry because they are available in major bulk options for incredibly low prices which makes it an easier & cheaper option for corporations, schools\universities, etc.
and because an average person\family can walk into a Walmart or Best Buy and easily pickup a cheap PC(s) without knowing anything about the subject.
not because of their hardware & build quality.
The power supplies are far better than consumer power supplies and are generally underrated if anything.
out of the 100 or so of these OEM systems i've worked on over the last decades none of them had even a decent reliable power supply included.
always a very minimal wattage(150-400w) with the lowest rating possible to help reduce production cost.
lacking suitable cables for any kind of forward upgrade.
and almost every time i've dealt with one of these systems that has totally failed, it is because the PSU has died during minimal usage scenarios.
Memory modules are very reliable brand names like Samsung and industry standard
almost every one of these OEM systems also always have had a very cheap, lowest possible frequency \ highest possible timings, single channel module that runs extremely hot.
The hard drives are also industry standard, so not sure how that speed is any lower than any other industry standard drive.
the average HDD included in these systems was almost always a 120\240\500GB 5400rpm drive.
comparing 5400rpm to 5400; wow you're right, same speed. amazing.
comparing to any decent drive; 10000rpm HDD, >500MB\s SSD, >2000MB\s M.2.
the speed differences are very noticeable in any situation.
of course now most give many options for adding larger capacity and faster drives, but they are still lower quality versions that will rely on the system's usual 1 year warranty.
What exactly is 'decent' hardware? Stupid gamer crap? I don't understand the mindset that ugly gamer crap is somehow better than reliable enterprise equipment that's actually use to run the businesses of the entire world? If the gamer crap was better, it would be the systems running the world. Again, something to keep in mind.
"decent" hardware is anything that is reliable, offers a good warranty, and offers noticeable improvements in workflow or any other use compared to "basic" hardware.
and again, basic OEM Dell builds do not include decent hardware.
you mention "business class" & "enterprise", so i think you have totally missed the point of this thread which is about purchasing basic systems and using them for gaming.
no one ever stated that Dell doesn't offer some higher grade versions of their OEM systems.
whether they be home "gaming" systems, science industry, or other enterprise use.
but that has nothing to do with class of systems that is being discussed here.