If you want to blame people for this, blame George Lucas and Steven Spielberg for making Indiana Jones and the Temple of Doom. That movie engendered the PG-13 rating in the States as a halfway house between PG and R. Though here in the UK we only saw a cut PG version in cinemas, the PG-13 rating opened up a space for films that had a bit more meat than PG offerings, but that didn’t carry the potential stigma (and the need to have a grown-up with you) that an R rating would mean. With PG-13 you get a compromise between violence and child-friendliness, and over time film companies have realised that that’s where you can maximise your audience for everything except explicitly family-oriented films.
That’s why all the major franchises (and may other productions as well) tend to target PG-13: the Bourne, Hunger Games, James Bond, X-Men, Star Trek and other franchises do this pretty consistently. The trend also has been for sequels to R-rated (15 or even 18-rated in the UK) hits to aim lower, ratings-wise in order to expand the franchise’s potential audience. See the later Die Hard and Taken movies for examples of this. You might make more money, but you risk making compromised movies that don’t appeal so much to the original fans (see the later Die Hard and Taken movies for this as well).
Also, we have a generation of moviemakers who, to a greater or lesser extent, have focused much of their careers making watered-down versions of the horror properties and tropes they loved as children themselves. Tim Burton and Guillermo del Toro (his US movies especially), I’m looking at you. Sometimes great movies are produced; sometimes we end up with Dark Shadows.