Why are the monitor vendors at fault when the cables have the poor shielding?
We don't know who produced these cables nor where they came from. I'd wager it's mostly an issue with cheaper products, and you often don't get any cables if you're buying a cheap monitor, so they're likely from a third party.
> Why are the monitor vendors at fault when the cables have the poor shielding?
Because part of hardware engineering is making devices that function even in poor environments, and that tolerate foreseeable misuse. (This is a major part of medical device development, for example.) Cable shielding is always iffy on consumer-grade cables. If other vendors can produce monitors that tolerate its absence, because that seems to be reasonably common!, then if you can't, your products will be seen as inferior. Exactly as is happening here.
Cable shielding often is subpar on industry equipment too. Yes, they have bought shielded cables, no they have not connected the shielding to the expected locations on patch up. Or just wrapped it loosely around, so everytime somebody patches up a new RJ45, you get a new quality situation, cause the vibrations shake the roulett box. Finally, ground.. ground is the yellow green cable and it goes to ground - and ground should be something professional done. Not a network of suffering machinery, pumping hovering potential into one another.
Also dont trust all electricians. Lots of them just try and error, nod in terror. Some do not even speak the language of the e-plan they are cabling up.
A bit of a problem is that HDMI is very sensitive to EMC thanks to DVI (which forms its base) being essentially VGA minus the DAC step - there's even porches and overscan included in the signal. But this meant that you could make very cheap TV displays, which thanks to very limited set of frequencies involved in cables could be made to work "stable enough".
So imagine all the problems you could have, and multiply it by variable frequency cable that thanks to increases in resolution has to run way higher clocks than it used to, combined with being cheapest possible design, and that unlike it's analog origins an error in bitstream can now crash the connection instead of just causing temporarily off-colour area.
Yeah, the simplest possible circuitry for driving a flat panel TV from HDMI (except the HDCP block, which AFAIK can be placed inline before the display logic) would be to declare the 720i/p capability and then just... pump the data straight onto display interface with some buffers here and there meaning to drop the porches, driven all by HSync/VSync signal recovered from the HDMI clock. :V
We don't know who produced these cables nor where they came from. I'd wager it's mostly an issue with cheaper products, and you often don't get any cables if you're buying a cheap monitor, so they're likely from a third party.