Because it works, is well understood, can be used by non-devs and there are tons of plugins that solve pain points. Not saying it is great or perfect, but for the average non-developer trying to stand up a marketing site, landing pages, blogging etc it is the smart choice until there is a reason not to use it. I dislike how it is built overall, not a big php fan either (however I respect it), but it works and doesn't distract valuable dev time to update some marketing copy or changing images or adding a blog post.
Even for developers, if they value their time and are starting a business it is smart to stick the marketing site on Wordpress. It means they can easily hand off tasks on it for lower costs and they can focus their energy on the important parts of the business that will differentiate them. Picking a less common CMS system won't differentiate the product or team, instead it usually just causes more headaches that are avoidable.
My general advice for anyone starting a company and wanting to stand up a basic site is to use wordpress. Build content on it, use it for what it is good at. And use your dev resources in areas that will make the business successful, which is not recreating the wheel.
To me it is the same as if you are starting a e-commerce business you are smart to use shopify, bigcommerce, volusion etc versus building a custom site to handle it. Even better to start with wordpress and woocommerce over some CMS + building & integrating your own commerce platform.
With anything there are exceptions, but just my 2 cents.