Because when people air their opinions, you forget the rest of their life, such as, are they OK in themselves, physically, mentally, are they happy, what expression is on their face etc.. are they sat with a coffee or a tea or maybe in bed with their partner? and instead, you focus solely on the opinion you're reading. If you dislike it, you may attack it, making it seem like you're attacking the person. In reality, as you say, it's not that hostile.
People read things how they read them, but that isn't to say that's how it's typed.
Reading a thread a day later can emit different vibes than you saw the previous day whilst being involved.
Debatign on the internet isn't for the weak hearted.