I mean like is there a good reason for that or is it thats the way it is?
A Brief Overview
WordPress offers three different options.
(1) You can edit your theme's css style sheet or create one of your own only if you purchase an upgrade and undertake css customization.
(2) If you don't purchase the upgrade, you can alter the appearance of your theme only through the use of customized headers and sidebar widgets. Note that if you do not opt for the upgrade then this link has information on point for you to read.
(3) Although some themes at wordpress.org and wordpress.com may appear to be the same they are not; they run on different software. The "please read me first before posting" sticky at the head of the forum outlines the differences between wordpress.com blogs and wordpress.org blogs which are on different code bases.
The two wordpress.com options.
(1) You can choose a theme from the admin area of your blog -> Dashboard -> Presentation -> Themes and select one as is. Many provide for a customized a header and sidebar widgets allow for posting of unique images, etc. to personalize your blog.
If you do not wish to use one of the themes made available at wordpress.com through your admin section -> Dashboard -> Presentation -> Themes then you can choose to undertake css customizing a wordpress.com theme or make your own, or you can move to self hosting and altering a free wordpress.org template.
(2) CSS customization can be used to improve on any existing theme; you can define your own classes for use in posts; you can select the Sandbox theme and build on one of the Minimalist layouts or, opt for no stylesheet and do it all yourself. If you feel you are up to the challenge of customizing a theme then these link, link, link will be helpful. There are also css resources that you can access listed in the FAQs blog.
The upgrade is not recommended to bloggers who do not have css customization experience. Although there is a forum and other bloggers may volunteer to help you if needs be, there is no staff support provided for learning css customization.
The 15$ you pay to upgrade a wordpress.com blog enables you to to alter the appearance of your theme and but not the underlying php code. Purchasing the upgrade entitles you to customize css on one blog for one year.
And if you are new to wordpress.com then this link will be useful to you because it contains links to the basic resources we use here.