There are several reasons for not entering the pattern from higher than standard altitude. First, that’s not what everyone else is expecting. Is anyone taught to look UP for intruders when checking for traffic prior to landing? In limited airspace and dense traffic, it’s everyone’s predictable behavior that leaves space for anyone’s improvisation in a true emergency. Your plummeting down from above could interrupt the studied plan of a ten-hour solo artist getting up the courage to turn downwind.
If you happen to be such an artist (or lack full currency, which can amount to the same thing) you need practice. Entering the pattern unnecessarily high would waste an opportunity to practice and refine standard procedure.
Other reasons: too many to mention. There’s a book on it.