If you're an adult who doesn't have or don't work around children it's hard to overstate how irritating they are. The noise, lack of self-awareness, the energy, the stickiness, and you have to censor yourself around them. Additionally, they're way harder to reason with (if they even can communicate) and parents are notoriously shitty and self-righteous. Finally adults have consequences. If an adult screams their head off, runs up and down the aisles, or hit people around them then flight attendants can do something

You also act like that child HAS to be on that plane. You can drive, you could leave the kid with someone else, you could take a train, a bus, or you could just not go. We know planes specifically are really uncomfortable for babies because of the pressure. You chose to have a kid presumably knowing that you'd be making sacrifices for them. Maybe one of those sacrifices should be flying until they're old enough to behave