“The Sith grew past the use of lightsabers. But we continue to use them, if only to humiliate the Jedi.”  -Darth Sidious

This also goes into why the Sith use different lightsabers.

They want to humiliate the Jedi by taking their signature weapon and altering it into something else. Changing it from it’s original finesse into something that is almost a satire. The more exaggerated the blade is, the more offensive. 

They also use synthetic crystals because they believed that the creation of one’s crystal to suite their needs was a show a strength.

“Although lightsabers are a superior weapon, there is still nothing quite as satisfying as feeling the warm spray of blood when one cleaves through one’s enemy with a real sword.”  - Komok-DA

In short, majority of the Siths’ behavior is not to make killing Jedi easier, but rather to make it more amusing.