You contradict yourself. On one hand you say pass rushers are easy to find. On the other you say they are spread out all over the draft. If they are spread out all the draft they would inherently be hard to find. They are extremely hard to find, hence their high value.
QB, WR, and planet prospects. Those are hard to find and rarely come from later rounds.
Pass rushers are found in every round of the draft, so [CORRECTION] hence it's *as easy* to find them as almost any other position except the above. A good amount of pass rushing skill can also be taught, so players who weren't pass rushers in college take to it in the pros when they get the right training.
Like most positions about half of elite pass rushers come from early rounds and the other half from the remainder of the draft.
Not so with QB, WR, and LTs/DTs. At these positions the elites are almost all 1st and 2nd rounders.