Here's why:
* History and Evolution: English words have a complex history, borrowing from many languages and undergoing changes over time. Sometimes, the spelling of a word is more influenced by its origin than by strict rules.
* Sound Changes: The sounds represented by "ew" and "ume" have evolved over time. While "ew" often represents a similar sound in both words, "ume" has a more unique vowel sound.
* Borrowing: "Spume" is likely derived from Latin, where the "ume" ending is more common. "Skew" and "spew" have a more Germanic origin, where "ew" is more prevalent.
* No Perfect System: English spelling doesn't follow a perfectly consistent system. Many words have irregular spellings, and these are often remnants of historical changes and borrowed elements.
In short, while it might seem strange, there's no single reason why "skew," "spew," and "spume" are spelled the way they are. It's a result of the language's history, sound changes, and the influence of different sources.