The seven mashkim are blood, water, milk, honey, wine, oil, and dew. What is the difference between water and dew? Isn't dew a type of water?
Yerushalmi Terumot 11:2 states:
רבי יוחנן בשם ר"ש בן יוחאי אם יאמר לך אדם שמנה משקין הן אמור לו הרי טל ומים מין אחד הן ומנו אותן חכמים שנים:
Rabbi Yochanan [said] in the name of R Shimon ben Yochai: if someone tells you there are 8 liquids [which can cause fruit to be susceptible to impurity], tell him that dew and water which are one thing, yet the Sages counted them as two.
His answer seems to be that they are the same thing, but they were listed seperately to emphasize the completeness of the listing, lest someone think that something which is not on the list but is similar (such as date honey) should really be included.
True, but it's not visible beforehand like water. If dew weren't listed separately, we might think that it doesn't count, because it just condensed from the air rather than actually "falling" on the food.
Besides, even water (or the other liquids) don't always make the food they touch muchshar (susceptible to tum'ah). Much of Maseches Machshirin discusses cases where water falls off something "shelo leratzon" (as an unintended side-effect of some other activity - one example is shaking a tree to knock off fruit, and meanwhile shaking water off it too), and if that water touches food, then that food still can't become tamei.
So the point of listing dew separately may be to inform us that even though it comes onto the food without human intervention (and, as above, seemingly from nowhere), yet as long as you wanted it to happen (e.g., you left fruit outside intending for it to become dewy so it'll stay fresh), then it is now muchshar.
There is water from rainfall and wells, and then there is water that has condensed from vapor in the air. The former is one kind of water, and the latter is a different kind, halachically speaking. The fact that they are chemically identical doesn't make them halachically identical.
For example, if one were to synthesize grape juice using chemicals and proteins, it would not be halachically considered wine. You couldn't make kiddush on it. See Ritva Pesachim 24b on Ta'am K'Ikar and Steipler's essay there, where a similar concept is explained regarding the taste of prohibited food. There are similar discussions regarding the cow that was created magically (Sanhedrin) or other meats occurring non-naturally, whether it is halachically meat, although chemically the two are identical.
A better question would be (and maybe this is what you meant), why is dew that condensed from water vapor in the air, different from rain which also condensed from water vapor in the air?
One could also ask, why is water from wells different from rain (as is obvious from laws of Mikvaos), isn't rainfall the original source of all well-water?
Once again, the answer is separating the halachic from the scientific. Halachah will consider well water different from rain water, and dew from rain, despite the fact that scientifically all water went through the same chemical process of vaporization and condensation.