In short, people are bad.

The most common reasons i have found are:
- They are very casual and didnt bother to read their tooltips properly/think its not worth it
- Melee is not their main class and they dont know how to play one (used to playing heal/tank/ranged dps only)
- Bad experience with early lvl tanks who spin the mobs, so gave up with positioning altogether in dungeons
- Just lazy