Since English is a stress-timed language, why have poets chosen to write in iambic pentameter? Doesn't the language already have a natural rhythm without resorting to meter? And isn't that natural rhythm already quite close to iambic pentameter?
(Note: It's actually a matter of some debate whether there really exist "syllable-timed" and "stress-timed" languages; but I think this question is answerable within its premise that there do, and that English is "stress-timed" — in fact, I think the answer is fairly similar whether or not that's the case — so I'll give it a shot.)
Unstressed syllables aren't silent; if the interval between two stresses is the same no matter how many unstressed syllables come between them, then stresses with more syllables between them will force those syllables to be said faster, which results in a faster rhythm.
So having roughly the same pattern of unstressed syllables throughout a sentence will result in a regular cadence:
- Liz Anne drank mixed drinks. (five stressed syllables, ´´´´´)
- Sally Mae had drunk a vodka tonic. (five trochees, ´˘´˘´˘´˘´˘)
- Amanda Jean had drunk a rum and coke. (five iambs, ˘´˘´˘´˘´˘´)
- Jessica Coleman was trying out something with grenadine. (five dactyls, ´˘˘´˘˘´˘˘´˘˘´˘˘)
- Alexandra was trying out something with peppermint schnapps. (five anapests, ˘˘´˘˘´˘˘´˘˘´˘˘´)
(Incidentally, note that not all of the cadences work equally well; personally I think the iambs and anapests work best, though that's probably subjective.)
By contrast, a more haphazard arrangement of unstressed syllables will have no such cadence:
- Liz Anne was drinking something with vodka. (´´˘´˘´˘˘´˘)
- Jessica Coleman drank mixed drinks. (´˘˘´˘˘´´´)
Of course, a regular cadence is not absolutely necessary to poetry. And iambic pentameter is far from the only possible regular cadence in English. (Common meter, for example, uses alternating lines of four and three feet; and limericks use anapests.) But it's a pleasant rhythm, and — as you imply — it falls within the normal range of natural English rhythms. (And of course, once a meter becomes common, it takes on a life of its own; later poets used it in part because they were in the same tradition of earlier poetry that had used it.)