MUSIC FOR THE ARTICLEIn many conflicts between good and evil, who is going to be doing the lion's share of being loud? The villains, more likely than not.
As for why, it's not known for sure. Maybe it's a reflection of their narcissism and total lack of inhibitions. Maybe they've decided that since they've already punted a few puppies, called down the lightning, and donned black robes, they may as well have fun with it. And maybe their evil actions are balanced by how much fun they are to watch.
When The Villian had his good guy facade on, he acted normally. When his evil side showed up, he started acting hammier. When he finally dropped the act altogether, he went to town.
While thought-speak can be directed at one, several, or even everyone in range, the Villian is CONSTANTLY BROADCASTING HIS PRESENCE to all, including his enemies. In fact, in the whole series, he speaks to one person at a time a grand total of ONCE.
The author sometimes does this intentionally for humor because the chief pleasure the aduience takes in his character is that it is almost impossible to imagine someone so extravagantly and flamboyantly evil.
In Star Wars This is actually something of an occupational hazard for Sith, since they draw their power from raw passion and self restraint is, consequently, not exactly on the agenda.
Many villains from 1970s and 80s Super Robot series in japan are this.
The Japanese actor Norio Wakamoto is famous for playing characters like this.