If you look at a candle in real life, you see a tall white stick with a yellow flame on top - but if you look at a photo of a candle you see a tall white stick with a white blot on top, possibly with lines sticking out of it, and maybe even a multicoloured lens flare if the candle is bright enough.
Since you’ve seen both of these all the time, you see both of them as “real” and “normal”, but if you were to see a candle in real life with an overexposed flame and lens flare you’d definatly not think it was real!
In the same way, if you were to see a normal candle with a yellow flame in a photo you’d think it was fake.
When you see a CGI render, the brain thinks it’s a photo, not a real object - so to convince the brain that it’s real and not faked, you have to make it look not real, but like a photo of a real object.
This means faking flares, focal blur, motion blur, etc - even though they make it look less like reality, they make it look more “real”.
So we have realism, making something look real, and photorealism, making it look like a photo of something real.