I asked that question when I was young. I was told that it was because we could only afford a house with three bedrooms, and my parents shared the same room so my brother and I could each have our own rooms. Even though I was a little kid, I feel like a smacked ass for buying that line. ROFL!
A few years later, my sister was born and my brother eventually was reloated to my room with me. I had to endure that nightmare for like 8 years until I went off to college and ended up moving out for good. As of 3 months ago, my brother got his own place and moved out. My sister is going off to college in September. Now, my father wants to remodel the house, add more rooms, etc. It's just him and my mother there now, but yet there is finally a pressing need for more room. That makes sense, doesn't it? Sorry, had to vent about this. It's kinda making me nuts.