For the most part, 75 meters (and a few places on 40/20) is where old, grouchy hams go when they've accomplished all they can in the hobby. They're done chasing DX, they hate talking to new people, and they're content sitting on their one frequency all day talking to the same old people discussing their physical ailments. It's sad, but true.
The honest truth of why this type of behavior happens in the hobby is because the Amateur Radio Service is low on the priority list of the FCC. Their first concern is public safety (Police, military, government, etc.), second concern is radio services that generate money (broadcast, cellular, ISP, etc.) Third is business band. Then you've got GMRS, FRS, MURS, CB, and Amateur Radio way down at the bottom of their list of priorities.
It's for this reason that the Amateur Service greatly policed by other amateurs. How it's supposed to work is reporting the interference to either the ARRL or FCC and wait. Sadly, those old hams decide that reverting back to kids and playing music, cussing, etc, is a better idea. It sucks, but it's how it's been for many years (decades.)
As much as I hate to say it, the best thing to do is "spin the big dial" and stay clear of 75 meters and other known childish frequencies. It's a terrible truth of the hobby, but just like everything else in life we have our good and our bad apples. Thankfully MOST of the bad apples stay in a handful of places on the dial, so the rest of us can enjoy the hobby away from all that mess.
Now side note, you mentioned 3.875 MHz. Now while it's only a gentlemans agreement and not a FCC bandplan, most AM activity on 75 M is on 3825, 3870, 3880, and 3885 KHz. With the amount of bandwidth that AM takes up, it's possible the AM group was on 3870 or 3880 and the SSB group on 3875 picked a frequency that was in use by the AM group. Of course, any voice mode is allowed from 3600-4000 KHz and is perfectly legal, but most courteous operators on HF know the gentlemans agreement bandplan and adhere to them.