Why is this hated so much?

I think it is blamed on the historical inaccuracies... and also that many people interested in what happened at Pearl Harbor are not interested in a romance.
But I think anything, fiction or not, that can personalize these kind of tragic events and can help people relate more on a personal level than military strategies or multiple death statistics ....then that's not a bad thing..

I saw it for the first time recently and I liked it well enough.
I liked Saving Private Ryan ,too, but aside from the topic of war as tragic I'm not sure I see the similarity.

The attack on Pearl Harbor was a well known and well documented event that was turned into a piece of fiction while Saving Private Ryan was a work of fiction loosely based on real events.

Historical figures like Kenneth Taylor, one of the pilots who got his plane into the air and shot down some Japanese planes, thought the movie sucked, for various reasons.

https://en.wikipedia.org/wiki/Kenneth_M._Taylor#Personal_life_and_depictions_in_film

The inaccuracies plus a lot of critics thought it was poorly written and the romance between the leads seemed shallow.
It's clear the movie was Disney's attempt to ride that wave left by not only Saving Private Ryan but also Titanic. I wouldn't mind a romance being included in a movie about Pearl Harbor but I think it was a mistake to make it the central focus.
Michael Bay might had been the hottest new blockbuster director in Hollywood at the time but he's not that kind of director;if they wanted an old fashioned romantic drama that happens to be a war epic then maybe someone else like Ron Howard or Robert Zemeckis would had been better. Someone who was more Spielberg-lite.
I was on the bandwagon to hate this movie when it came out but I've softened up to it. The romance still sucks and it's too long though. But I will say that the thirty minute Pearl Harbor attack scene is actually pretty well directed. Both the visual effects and the action holds up.

It tried to be a Titanic love story Ripoff centered around a tragedy related to War that didn't go over well. It's also very boring.