Keeping so many aspects in the mind the core development team has to keep the style as simple as possible. Don't think the team cannot make fancy themes available in the market. They just want to make it simple so that every type of website can use this theme anytime without breaking the site layouts. But the default themes can be painted to any design. You can look at our site WPspice. We are using the basic Twenty Twenty Theme. You will get the idea of what can be done with the default themes. That's why the team keeps the design neat and clean and simple. As the twenty twenty-one theme will come with unique block patterns so that you can easily create elegant websites with the help of the block patterns.