system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are special effects/CGI becoming better? Is there a point at which CGI just can't become any better/more realistic?
gpt
The problem CGI is facing these days is called 'the uncanny valley'. It means that, even though you *consciously* are unable to remember exactly where the freckles on the skin of your significant other are placed on their skin, or the way their eyebrows are curved, heck, even the way their hair does look a certain way every morning when they wake up, so anything like those small details, you *unconsciously* still 'know' the way it has to be, in order for them to look 'right'.

Now, obviously you know a person you're sharing a great deal of time with (family, friends, etc) very well, so this comes to no surprise. *But* this effect of subconsciously 'knowing' how a human being has to look, expands to completely unknown people, animals or objects as well. Take "Monsters, Inc." as an example. They needed megatons of animateable hair for all those furry creations they had (most well-known: James P. Sullivan) in the movie. They *invented* new tech just to do that: Render and animate billions of single hair. And yet, even though you obviously never saw a being that looks anything like Sullivan, you still would see that the flow of his fur was a slight bit 'off' as in "That's not real fur!".

Over time, technologie got much better, to the point of some remarkable success in fooling your brain, specifically in fast moving environments and with inanimate objects(Think 'The day after tomorrow' quakes, collapsing houses, crashing cars, etc, all from rather far away - these kind of things work pretty good), but as soon as you get into the area of living things, moreso in closeup shots (e.g., the avatars from Cameron's "Avatar"), the uncanny valley effect kicks in and your brain goes "Now, wait a second, *something's* wrong with that car/tree/cat/person.." and you immediately see through the cgi works.
It will take quite some more effort until cgi can produce a dog or cat (...) that our brains will accept as 'real', even more with a human being. It might happen some time in the future, given new technologies and/or software.

tl;dr: Brains are not easily fooled to consider a cgi 'thing' a real object or person. The more familiar your brain is to a given 'something', the harder it is for CGI to produce said 'something' as lifelike enough, to overlook the fact it *is* CGI - and vice versa, the more foreign or alien (sorry) something is, the more willing your brain is to accept it as real. As of today, the technology is not there, yet, but this may change in the future.

(Edit: spelling, grammar)
(Edit2: More spelling)