Another reason is that PSUs, like many other computer components, and products in general, are being made as cheaply as possible. Design, quality, etc. are declining and with the PSU being a critical component I tend (rightly or wrongly) to consider the PSU as a primary suspect in these sort of situations.
While there are cheap PSUs out there, with questionable quality, select few PSU OEMs are still upholding their moral standards and produce good products. Those few are: Super Flower, Seasonic and Flextronics. Sure, other OEMs also have good PSUs in their lineup, like HEC or CWT but since they have 0 issues producing low quality PSUs, i don't hold them in that high degree of trustfulness.
Here, i don't think the situation in PSU scene is due to the OEMs. Instead, it's due to the penny pinchers who cheap out on PSU. As long as people are buying cheapest PSUs, OEMs are making them.
Since i'm specialized on hardware, i 1st look if any of the symptoms are most likely due to hardware issues or software issues. E.g USB disconnecting is usually an OS issue, while PC randomly rebooting is usually PSU issue. And once the split is clear, i follow the route to the cause.
Regarding software/OS issues, yes, many people don't have their data backed up and wiping the OS + installing it again will (for the most part), erase personal data as well. But on the other hand, this is very good lesson for people to; either back up their data OR keep their personal data on another disk. (E.g i have 3 disks; OS, data and backup.)
It is possible, after tedious work, to locate and fix the software/OS issues. Only in the rare cases, when OS has corrupt in high degree, where only fix is OS reinstall, there is no way around for saving personal data. Though, for most cases, even when OS is corrupt, but the drive itself isn't, the drive can be connected to 2nd PC as data disk and all personal data can be just copy/pasted over (i've done it several times for my friends/family).
Though, i'm not software expert and i don't like to deal with software/OS issues, hence why i suggest OS reinstall. It's faster and removes any software issues PC may have. A clean start to say so, from where to look further what the issue may be (usually, issue then is hardware related).
On hardware side, things are easier (at least to me). And there are several steps that can be done to test if the hardware is sound or not. But in most times, those tests require 2nd PC and/or replacement component.
Yeah, I don't suspect the PSU is the cause of this problem, but since it ought to be eliminated anyway because it's inappropriate for an RTX 3070, it's a no-brainer to replace it and eliminate at least a possible, though less likely, source if issues.
The thing with prebuilt PCs is, that the PSU is often overlooked and cheapest one is used, which doesn't have the quality, max output wattage or reliability to run PC safely for years to come. Only thing that most people are interested in, is cheap price.
Depending on where prebuilt is bought, sometimes people can make a choice regarding which PSU it comes from, but for the most time, cheapest PSU is still picked. I guess it isn't well known fact that PSU is the most important component inside the PC, because it powers everything. And if low quality PSU goes sky high, PSU has the magical ability to take everything it is connected to, with it. Resulting in completely fried system, where only fix is buying a whole new PC.
Saving $40-$50 on PSU isn't worth the $1000-$2000 people need to fork out for a new PC, once the cheap PSU fries their current PC.
Based on your comments I think I will upgrade the PSU anyway as I noticed that the builders (it was a pre-built) had looped a single 8 pin power connector into both the GPU power sockets.
Can anyone recommend a good PSU?
it's good to hear that you got your issues solved.
Regarding prebuilt PCs;
I understand why some people are buying prebuilt PC, rather than buying separate components and assembling them on their own. For the most part, is lack of knowledge in PC assemble and there's always they laziness factor as well. It's far more convenient to pick up prebuilt PC, hook it to monitor/ KB/ mouse and start gaming, rather than spending several hours assembling it.
Though, GamersNexus has reviewed several prebuilt PCs and every single one of them has come with issues. Some with more severe than others. So, the best course of action is to inspect your PC thoroughly when it arrives since it may have loose connectors, broken hardware etc.
As far as which PSU to go for, i suggest Seasonic Focus or PRIME lineups, in 750W range,
pcpp:
https://pcpartpicker.com/products/compare/64cMnQ,dCs8TW,VfM323,cNsmP6/
(All 3 of my PCs are also powered by Seasonic, full specs with pics in my sig.)
For other options:
Super Flower Leadex II or Leadex III
Corsair RMx, HX, HXi, AX or AXi