Activated theme looks nothing like preview

I am very confused as to why when you activate a theme, it looks absolutely NOTHING like the previewed theme. Is the preview of the theme basically a preview of what could POTENTIALLY be done if you modified a ton of the html/css?? I am pretty appalled that there are no instructions/explanations about these discrepancies…

The current theme I’m using is edin. Their “preview” looks amazing and when I activate it, mine looks like a really crappy blog post template, not at all the full website looking version of their preview. Help please!