Why are professors' websites so ugly?

Not ALL professors' websites are terrible. As referenced in this nature commentary, many academics acknowledge the potential benefits of having a good website. This article also links to a contest where submissions for the 'best lab websites' were solicited. Many of the links there are great examples of lab websites with elegant aesthetics and intuitive navigation.

I think a lot of this boils down to the fact that many faculty are older or too busy; making a good website takes time and skills that most academics simply don't have. Some labs will hire web designers to help with design, but today's funding climate makes that impractical for most PIs.

Having a good website for your own interests and professional development can only help you. I can't see any downside to this...

There is another aspect that I have not seen mentioned in other answers.

In some scientific fields/cultures, a stylish website could be viewed as unnecessary or even pompous. In this view, the textual content of a website is the only thing that matters, and if you "need" to make your website stylish perhaps it lacks real substance. This is the same line of thought that supports simplicity in presentation with minimal graphics. I have encountered this especially in math and theoretical CS.

There might also be a prestige factor, along the lines of "I am so important, my work is so well-known and I am so busy, that I don't need a website".

In many other cases I agree it is simply a lack of knowledge/time/benefit.

Because there is no need for them not to be.

Professors and academics, per their job descriptions or the scope of research grants, mainly have research, teaching, and students supervision duties, mitigated by administrative hurdles. None of these are addressed by polishing their website.