I don't quite understand why sexually promiscuous adults get tested for STDs unless they show symptoms. Or even why they freak out about them?

Just go with the assumption your partner ALWAYS has an STD. They almost certainly have HSV-1 and a damn good percentage have HSV-2 and many don't even know it, as they are asymptomatic. You can transfer oral HSV to genital HSV and vice versa, and you can do so without knowing you have the condition and without active breakouts.

Then we have the big one: AIDS/HIV+. (Which actually is much better than it was 20 years ago. 10 years from now we'll likely not worry about this one too much.) There is no way with absolute certainty to be able to determine with this infection someone is 100% clear up to a year after they have a sexual encounter. False negatives are possible up to a year.

Then we have the fact that people lie/cheat. A lot. More than you think, combine that with the multiple STI/STDs not even mentioned that are frequently asymptomatic, and the presumption now is ALWAYS ASSUME YOUR PARTNER HAS AN STD. Period. End of discussion. Get tested for your own sanity, or to clear up your partner, but more sexually active people have STDs than don't.