This is why TakeShape also provides a built-in static site generator[0] using Nunjucks templating (think Django, Twig, Swig, etc). If you are trying to build a simple site this is a great option. The great part of a Headless CMS is that you can choose whatever frontend you want and your solution can evolve with your product.
It's a bit like asking why you need an editor to read your manuscript or why you need a typesetter to typeset your book (instead of just saving a Word document in pdf and sending it to the printer).
Take a look at Smashing Magazine, for example. That's a static html website built with Hugo. Guess who built it? Professional frontend developers :-)
Maybe you don't need a professional for your site, or maybe you do. If you are a company that is sold on the idea of having a very fast, elegant, interactive, and yet searchable site, then maybe you do. Also, if you are a frontend developer and already have the skills, then maybe you'll like this tech for your own site (I know many frontend devs do).
- knows modern JS - knows ReactJS - knows webpack well - knows how to deal with node_module dependency hell - knows graphql