Please tell me why people use comparative in this sentence. It is very strange to me.
The parents were more happy than surprised at the study result of their son.
Why don't they use "happier than surprised"?
Here you're comparing two descriptions, happy and surprised, saying one is more suitable or accurate than another. -er is not used in such cases.
He's more lazy than stupid. (NOT He's lazier than stupid)
Practical English Usage, Swan, p. 114