"In the house" is not a complete sentence. It leaves you hanging. In Arabic, this incomplete structure is called Shibhu Jumla (A Phase or "Sentence-Like" structure). Let's see how prepositions create phrases and force nouns to take a Kasra (`ِ`)!
When a preposition (Harf Jarr) comes before a noun, the noun is mathematically forced into the Majroor state, ending with a Kasra `ِ` (the "ee" sound).
A phrase cannot stand on its own. But if we add a Mubtada' (Subject) to the front, the phrase happily acts as the Khabar (Predicate) and forms a complete sentence!
"On the desk..."
What is on the desk? The listener is still waiting for you to finish!
"The book is on the desk."
The noun is the Subject, and the Phrase gives us the Location (Predicate). Perfect sense!
Let's review the 4 prepositions. They act like anchors, dragging the noun that follows them down into the Kasra case.
Read aloud. Notice how the prepositions force a beautiful Kasra `ِ` onto the nouns (Mosque, House, Desk)! No exceptions here!