A team paying a pitcher $5 million for 180-200 innings gets more value for their money than one paying the same amount for 70 innings. So it stands to reason that so many teams are insisting pitchers stay in the rotation as long as their performance is tenable.
Unfortunately, this means many teams miss out on potential dominance from those pitchers over shorter stretches. For some, this is due to the restraint needed to stretch one's effectiveness out over 100 pitches. While they may be dominant giving 100 percent, they can only give 80 percent to last that long.
For others, they have only two effective pitches, which is enough for one or two innings, but starters usually need three to turn over a lineup twice. These pitchers would be much better off used in the 'pen, but of course, the value of raw innings pitched will keep them slaving away in the rotation.