It does though. Because for something to be challenging it will inherently take longer. If you have to stop and consider a pack of mobs it takes longer. If you have to plan your route to a quest objective it takes longer.
Tougher mobs to kill does not teach new players how to use their character. The way you play while leveling is different then how you play end game. Yes you learn the basics but a competent enough person can learn that at level cap, or through the current expansion leveling process. Guides and videos make it even easier. Challenging leveling is only exciting if you like a challenge. After you've done it for X amount of times it is no longer exciting.
Leveling shouldn't be challenging. Having to wait for a group to do X quest wasn't challenging or exciting back in the day. It just meant that you skipped that content or got frustrated when you couldn't group up. Having to ride your mount or take some way off course route because an elite was randomly mixed in the quest area wasn't challenging.
There are plenty of ways to improve the leveling experience but making mobs take longer to kill isn't exactly challenging. And new players can't be challenged to much. Existing players will never find normal leveling a challenge since they are used to end game (and are likely using heirlooms to deflate the already low difficulty).
For example making leveling more scenario like and co-op or small group friendly would drastically improve leveling. You can offer different types of challenges and encounters while still making it quick process. No one should have to be challenged through 100 levels in order to face the content that is designed to challenge you.
Besides its not like this change has anything to do with what you want from leveling. It is changed to keep a sensible progression through the zones story. The concept of zone stories you progress through while leveling is outdated. Scaling helps but you still need to increase mob difficulty to keep the flow.