Because otherwise, a piece of malware that installs itself at a "mega-privileged" level can easily make itself completely invisible to a scanner running as a low-priv user.
Heck, just placing itself in /root and hooking a few system calls would likely be enough to prevent a low-priv process from seeing it.
My argument was that this is the flaw, and everything else is just trying to put lipstick on a pig.
If you have a nightclub and you have problem controlling which people get in, the first idea would be to not have a thousand unguarded doors and to then recruit people that search the inside of your nightclub for people they think didn't pay.
You probably would think about reducing the numbers of doors and adding effective mechanisms to them that help you with your goals.
I am not saying we don't need software that checks files at the door, I say we need to reduce the number of doors leading directly to the nightclubs cash reserve.
I don't know about PNG, but I'm fairly sure JPEG works this way. You can concatenate a JPEG file to the end of an executable, and any JPEG parser will understand it fine, as it looks for a magic string before beginning to parse the JPEG.
A JPEG that has something prepended might raise an eyebrow. A JPEG that has something executable prepended should raise alarms.
If there is any place that historically was exploited more than all other things it was broken parsers. Congratulations if such an exploited file is now read by your AV-software it now sits now at a position where it is allowed (expected) to read all files and it would not surprise me if it could write them as well.
And you just doubled the number of places in which things can go wrong. Your system/software that reads a PNG image might do everything right, but do you know how well your AV-software parses PNGs?
This is just an example, but the question we really should ask ourselves is: why do we have systems where we expect malicous files to just show up in random places? The problem with IT security is not that people don't use AV software, it is that they run systems that are so broken by design that they are sprinkled on top.
This is like installing a sprinkler system in a house full of gasoline. Imagine gasoline everywhere including in some of the water piping — in the best case your sprinkler system reacts in time and kills the fire, in the worst case it sprays a combustive mix into it.
The solution is of course not to build houses filled with gasoline. Meanwhile AV-world wants to sell you ever more elaborate, AI-driven sprinkler systems. They are not the ones profiting from secure systems, just saying..