When it comes to 'landing pages' (where describing the purpose of a company or product is the goal), I think the problem is primarily that it's just really hard to do right, content-wise. The visual design is at most a secondary problem.

A more general-purpose home page has it's challenges, of course, but often it's really just about showing something pretty and guiding users to pages such as 'pricing', 'buy/shop', 'contact', 'about us', etc.

A landing page, on the other hand, is basically the same as creating an ad or a commercial. You have a limited time to 'sell' something, and the fact that you need a page for it means that it's not immediately obvious what you're selling. You have to write ad copy, consider the visuals, target audience, selling points, etc. This is much more difficult than just having a few pages that do very specific things, and a home page that points to them.