The difficulty levels of dungeons increase as you go along. As you advance, you run into more and more foes in encounters. Any dungeon master who creates their own adventures, and started with level 1 dungeons, can tell you it’s hard not to kill the adventurers when they begin. You place small amounts of weak enemies in adventures, and this principle applies to DDO just as well. But as you advance, this changes, and after a few levels, a group of adventurers can handle quite a bit.
When soloing in DDO, this will be the big problem you run into. Certainly you can handle the small amounts of foes at first, but after level 2, you will run into larger and larger amounts of foes. Soon, some of those foes can damage you fast enough that even when you’re killing them off as fast as you can, you are killed by the rest, whether by spell, arrow or sword.