Why use HTML and CSS when there is WordPress?

Apart from what Peter Drew said, Wordpress can have many disadvantages over a custom-coded website.

For example, you are limited to whatever themes Wordpress has to offer, and some of them can be really hard to implement. With a custom site, the sky's the limit! As a side note; if you know CSS and HTML you could also modify an existing theme to fit your needs.

When you want to make a website it's important that you ask yourself what you want the website for and who's going to use it. If you only want to use it for a 3-page "portfolio"-site, then wordpress is probably more than you need and your site would run slower than necessary. If the website is targeted for larger audiences, then wordpress is probably the best choice because of it's superior flexibility and security.

I have personally never preferred using wordpress over a custom solution. Even if Wordpress is extremely modular and flexible, I still feel like my hands are tied. I want to be able to able to make a website look and behave the way I want it to -- unfortunately at the cost of time of course. Sometimes you may want to develop a website in which Wordpress cannot be used; for example, if you want to create a web-based control-panel for a specific system, or maybe you want to make another e-learning platform like Treehouse ;-) In neither cases can you use wordpress to fullfill the functional requirements of such websites.

Though, if all you'll ever do is to create a personal blog, or a family website, or a website for your shop, then 9 out of 10 times, wordpress will be more than enough!

So is it worth learning HTML and CSS over only Wordpress? Absolutely (it's more fun too!)

I agree with what was said above, so I'll add.....somebody had to learn HTML and CSS to build the themes for everyone else to use. :)