Why do some people build loads of brown houses, they barely make any money..

Brown Houses make $6 every 5 minutes, as long as you keep clicking them, and cost $355. That's 5 hours to pay for itself. Purple Houses make $75 every 4 hours, and cost $1100. That's 60 hours to pay for itself (long enough to earn $4,320 from a single Brown House, if you could automate it somehow). The more time you spend actually playing, the more reward you get out of short-time rewards. Lots of Brown Houses give you a much higher payout while you are playing, but less payout when you are doing other things.

The ones you can leave for long periods of time earn you less Money than the time-intensive things.

Cuz they're cheap and improve you're indolence on the confirm-o-meter