It seems from the various answers given, that it's really a combination of factors.
1. Path of Greatest XP
SC (Skill Challenge) and Combat give the same XP so that players and GMs don't choose one style of play over another based solely on XP gain. This allows the mechanics to play a secondary role to the story or campaign. It also explains why traps are the same XP value as Monsters.
2. Character Time
While combat might involve more risk of life, a SC takes more time from the characters' point of view. Thus the amount of effort between a SC and Combat, from the characters' view could be argued to be roughly equal.
3. Player Time
A well run skill challenge, will in fact take just as long to play as combat, and can be just as taxing or engaging on the players.
4. Character Effort
A dangerous trek through the swamp, or subduing a wild crowd can be costly to a character if failed. (i.e. losing a healing surge is a mechanic for a failed Skill Check) Therefore in many cases, a SC will in fact require the same amount of effort or heroic actions from the character as a normal combat encounter.
5. Player Effort
To overcome a SC, a player will usually attempt to use their best trained skill. To get that best trained skill to be applicable to any given skill challenge, can be just as rewarding or difficult as picking the correct tactical position and power during combat. That is, the player must be creative to find a way to have a History roll be applied to tracking a monster.
I'd just like to note that I think the rules compendium doesn't make it clear enough that a skill challenge can have results such as lost healing surges, or deadly combat as a failed result. That is, the mechanics as described in the books don't expand enough on the possibilities of engaging and interesting skill challenges.
Is there anything I'm really missing?