The PG-13 and R rating split is lower the quality of films. The movie industry is forced to cater to 13 year old teens instead of a more obviously mature 15 year old target demographic, the largest one that sees most big blockbuster movies.
Due to these constraints for teenagers the rules get really silly, like the one F word limit on PG-13 films, not allowing any type of nudity and limiting violence to cartoon depictions.
There are exceptions to this rule, some films tend to adjust to the PG-13 rating and still achieve darkness and grittiness, but the compromise to art for a PG-15 demographic would be considerably less substantial.