Hitting doesn't just deplete the opponents' hp--its also necessary to add status effects to enemies, or removes status effects from allies. Hitting triggers healing, allows the attacker to teleport or stop an enemy from teleporting. Hitting allows pushes, pulls, slides.
In complete agreement. To go one step further (in case it's not obvious), hitting more often means your opponents die faster and in the period before their defeat will thus deal less damage (and other effects) to you.
The same argument goes for defenses, of course: the better your defenses, the longer you survive, thus the more damage you can deal. The difference is in the "details": it's quite feasible to maintain a hit ratio of 55% (and situationally much higher), whereas it's virtually impossible to maintain decent defenses, certainly if you consider NADs which (even if you take improved defenses and otherwise focus on them) will generally be trivially hit by monsters.
Scaling defenses is also more difficult because it involves 4 stats, and almost no rule elements boost all four. Tactics also plays a role; 4e is quite team-play and combo focused, and pulling off a groups best combat tends to require a series of attacks - if one fails, the rest are weakened too. So there's a kind of multiplier going on: the more accurate your attacks, the more elaborate team plays you can make. On the monster side of the fence, the same is generally much less the case for various reasons. Focusing on attacks over defenses reduces combat length; again, a factor in PC's favor, who have a limited number of encounter/daily powers that don't recharge and represent a greater improvement over their at-wills than monsters' recharge powers. So as the combat takes longer, the scales slowly shift to the monsters' favor: if you can't win using dailies and encounter powers, then by the time both sides are down to at-wills (and the monster to occasional recharge powers), the PC's have lost more.
Then there's the fun! It's just more fun to dramatically and swiftly kill things rather than merely die so slowly you win by default. It's fun to build the occasional tough-as-nails tank valuing survivability over offense, but most characters are more dependent on their active abilities, and while one defensive turtle in a party can be cool, as a strategy for an entire party, it's not fun.
So, there's truly
lots of ways in which 4e encourages focus on offense over defense. When it's easy to get or critical to your role/character concept, defenses make sense, but that's the exception, not the norm.