for one, licensing fees, perpetual upgrading that at times requires you to buy the next version.

you can buy linux distros, but most of them have online checking programs that look for updates, patches, etc.. and they do not charge for this.

it is also possible to simply update to the newest version of your distro online as well, just download via ftp the corresponding update packages or if its a source distro, install and you have an up-to-date system.