Why do they make movies where the music is so loud that you can't hear the dialog?

More than likely, the imbalance between the music and the dialog is sourced in the local playback equipment. If this is your home equipment, try adjusting the surround sound settings. If it was at a theater, mention it to the management and hopefully they'll fix it.

If it was actually in the movie, then it was either done intentionally as a way to get you to try to pay extra attention to the dialog or was the result of poor work by the sound engineer.

If you listen carefully then you should be able to understand what's being said. Without music movies can feel really low budget and boring. Some of the biggest blockbusters had a soundtrack. In my opinion every good film has one.

Because they think the music sounds better than the dialog.

That infuriates me   it is so stupid and annoying

cause some people like loud stuff