Why is 4e combat this way? Well, because of the features of AD&D (1e and 2e) and 3e combat.
The feature about low-level AD&D combat that most comes to mind is "random". Yes, skill can mitigate luck to some extent, but it doesn't take long to put you on the ground, bleeding to death: just a couple of (un)lucky rolls. As the game gets into the higher levels, this damage randomness decreases, but instead the game hits the "one bad save and its over" swinginess.
I would say there are very few DMs who didn't fudge combat in AD&D - particularly low-level combat - so that PCs survive where they shouldn't have.
Once you get to 3e, you get that in spades, added to which was an advancement system that didn't have proper checks and balances. It was entirely possible to game the system: spell DCs that no-one could defend against except for the defense optimizers that no-one could touch.
At the higher levels, you were looking at one spell taking out all the enemies, or often no more than two rounds of combat. The problem? It'd take an hour or two to resolve those two hours! Swinginess and extreme length hit high-level 3e.
4E combat is designed with two major goals in mind: reducing the randomness of combat and stabilizing the length. Most combats should take 40-60 minutes to resolves, and last somewhere around 4-8 rounds.
It succeeds at the first pretty well, IMO. The second, not so well, although most of my combats have lasted about that long until recently (and we hit 18th level!)
Why does the second fail? The reason comes with an overestimation of the defenses of the monsters. Take an Elite Soldier of 2 levels higher than the PCs. At this point, the PCs have something like a 25% chance of hitting it. This is where the major problem of grind comes from. This soldier is (by the guidelines) something that the PCs should be handle - and, in fact they can - however, the combat takes so stupidly long that it destroys the pacing of the session.
(Stupidest monsters ever are those with insubstantial and cause weakness in the party. That's horrendously bad design).
Higher levels monsters in 4e are more dangerous, yes, but they also take substantially longer to defeat. If you check the Wizards adventures, they've got entirely too many combats that are more difficult than the PCs, and not enough easy combats.
Even with all of this, my 18th level combats in 4e are taking substantially less time to resolve (and with a lot more actions per character) than my 16th level combats in 3e.
In every single edition of D&D, I've needed to kludge combats. Not perhaps every combat - I've run a lot of it by the book - but there are times when the system just falls over.
Do I think the basic length of combat in 4e is too long?
Certainly I do! However, the game still works pretty well for me until we start getting these 25% chances to hit. A little less defence, a little fewer hitpoints... 4e wouldn't need all that many changes before it ran combats in a more reasonable time and didn't have so many trouble with over-level monsters.
There is a secondary problem with the game getting a little too condition-happy (and thus trickier to track and slower), but it isn't as significant as the defenses issue.
Cheers!