Hi,
nobody forces you to use Stylify and the same arguments, you write against utilities could be used against vanilla CSS.
From your perspective, it might look the way you wrote. Yes, Bootstrap is a well-known component framework, no doubt about that. But that doesn't mean, new tools cannot be created, otherwise, we would still be stuck with for example jQuery.
Stylify is inspired by Tailwind, it has similar features but also those, that Tailwind doesn't have, like splitting, mangling, smaller CSS bundles, it's faster and easily configurable. Yes, it doesn't have a such community or color palettes/themes, it's a new tool and framework palettes often don't make much sense as they don't match project needs.
Vanilla CSS is a great way to style a website. It's the "end product" of all frameworks. However it's not that simple to write it in optimized way and frameworks, like Stylify and Tailwind, automatize and simplify some work you have to do while working with the vanilla CSS (duplicates, unnecessary high specificity, purging, optimization, reusability) https://stylifycss.com/docs/get-started/why-stylify-css.
But I understand that you might not see it as a benefit. In the end, use what you like.