There could be a few reasons
1. Make sure your installation of Excel is up to date
2. Is your Excel version a Student or special version that could have that option disabled?
3. Try going to the Start Menu, Control Panel, Programs and Features, Scroll down and find Microsoft Office, Select Change at the very top, go through all the options and see if Sparklines was an Optional Installation? Correct as appropriate. (I doubt this is it, but worth checking)
4. If your on a corporate system Check with the IT people, some companies are paranoid about using new features until they are years old and tested by everybody else
5. Your installation has become corrupted somehow, uninstall and re-install Excel