Why do people judge others by their looks? Like what clothes they wear, what kind of a house they live in etc.

Firstly, not everyone is like that.

Second, It's human instinct to guess the unknown in order to fill one of the biggest human's desire which is "curiosity". 

Take the clothes for example. When you see someone with dirty or messy clothes an unknown subject with trail of guesses will shape in your mind. 

Maybe he doesn't have financial power to buy new clothes? Maybe he's rich enough but just prefers to go around like that? Maybe he's just a normal guy who lacks personal cares? Maybe recently he's been depressed & avoided self care? Maybe the moment we'd seen them that way they just been involved in a fight, accident etc... 

We all make guesses & judge in that way, whether we realize & accept it or not.

The other type of judgement is giving worth & good personality by what people wear, how they look, & where they live. 

In simpler language... some people judge others by how much their clothes & houses & places they visit actually cost. I don't have anything to add about those type of people, I only call them unwise, unthinking & materialistic people. I've never been one of them. 

But I'd guess that's also one of unstoppable human's instincts. Especially since our lives on earth are moving everyday towards a more currency-based & materialistic system. 

And yet there are so many types of people who are fighting against these prejudices, and so many things that can't be bought with money:

1. Manners 

2. Morals 

3. Character 

4. Trust 

5. Patience 

6. Integrity 

7. Respect 

8. Love

Most people do it because the way someone express themselves. Often it says a lot about them, same with how they take care of things and how much they spend on things. Not everyone will judge based on appearance and material objects, but it does happen very often because people feel it's a good indication of someone. 

If I went to someone's house and it's very nice, looks expensive and clean then I can see that person has money, they are organized, they take care of their things. If I go to someone's house and it's dirty and has trash everywhere then I may not want to come over or trust them with my things because I can see they don't take care of theirs. 

In the same way, people judge appearance for similar reasons because they want to know who they are going to be around and talking. A first impression can say a lot about someone, like in a job if you come in with dirty clothing and unwashed hair it says "I don't care" and people see that, even if in reality you were just having plumbing problems and couldn't do laundry and shower.

Because humans are a visually-oriented species.

Because we live in a judgmental world. How else can I put it.