Why is the down payment on a house so high?

I'm in the working of looking for me a place of my own since I know have a full time job and saving money. I'm not educated in this stuff so i have no clue about this. Now i'm looking at some houses around (I'm a 24 year old guy btw) town, some that are to buy i've found have some mortgages i can afford between ($250-$400) that can be done based on how much i make a month since i only get paid twice a month. But the down payment is outrages one house i found had a $241 dollar a month mortgage but a $20,000 down payment, like how can someone afford that? I mean like i have no where near that much. Most places around here have crappy houses and apartments. I'm looking for something nice but not fancy and in a decent neighborhood. How does any afford that? Call me whatever you want, but i'm just trying to understand all this because my parents have no clue, which sucks because i'm left in the dark in everything.

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