In this post, "Parshablog" notes the great similarity between several midrashim on frogs and verses in Shemot which describe the Israelite's experience in Egypt. As one possible explanation, he suggests that the rabbis in the midrash wanted us to see the plague of frogs as punishment for the oppression of Israel. Thus, they told various additional stories about the plague, which were not historically accurate, but serve to highlight the thematic connection they were drawing.
But where else do we see midrashim intended as literary parables, in which the authors make clear that the events in them did not actually happen? My impression is that's rare to nonexistent, much as it would make understanding midrash easier for lazy people like me.
Rather, let us note the pshat basis for saying that the frogs were punishment for earlier sins. The Egyptians tried to prevent being overrun by the Israelites' growth ("vayishretzu"), and they were punished by being overrun by frogs ("vesharatz"). They tried to throw the swarming Israelites into the Nile, and as punishment frogs swarmed out of the Nile and overwhelmed them. On a pshat level, the Egyptians were likely supposed to look at the content of the plague, and realize that it was a measure-for-measure punishment.
If so, then all the midrash adds is the assumption that the measure-for-measure-ness was complete and exact. With that assumption, it takes every available piece of information about Israel's experience, and concludes that the exact same thing must have happened regarding the frogs.
Why make this assumption?
Both modern and medieval Jews are often forced into a corner somewhat regarding our religious beliefs. The Rambam, based on his understanding of philosophy, was forced to say that many stories in the Torah (i.e. Bilaam's donkey talking) were allegorical. We are forced to say that the world was not actually created in six days. Pashtanim in general are forced to say that sometimes Tanach was grammatically irregular or imprecise. In all these cases, outside factors override what seemed to be the Torah's position, but we are willing to compromise since we believe that enough of the "core" issues remain unaffected.
Perhaps the distinguishing characteristic of midrash is that it is never willing to make such a compromise.* It is never willing to admit that a verse (or letter) of the Torah cannot be understood and thus is invalid as a source for halacha. Similarly, it is never willing to limit the number of miracles in a story based on considerations of reasonableness. Thus, once it decides upon an interpretation such as "the plague of frogs was a punishment for the attempts at population control, and the punishment should fit the crime", it proceeds to insert every single detail of the crime into the punishment. Once we have decided that the Torah's message is that the crime and punishment are related, how can any external factors impede or limit what the Torah is trying to convey?
*By the way, I think it's too simplistic to say that charedim today follow this approach. It might be more accurate to say that Chazal claimed to know everything, we claim to know something, and charedim claim to know nothing and thus repeat the Chazal's statements without presuming to fully understand them.