What is the reason for some very popular sites like Reddit use ugly User Interface? [duplicate]

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I got a project to build a website like Reddit. My client need a very attractive modern look interface.

But he asked that why Reddit website look so ugly? IS there any UI /UX reason for it?

I did not have a answer when he asking. I said I will do a research and let you know. But still I could not find the reson for it...