I learned it at school as a kid. Not long. But I've taught others, seen adults pick it up. Like most other skills, it just requires work and practice. Immerse yourself in it, want to learn and you will. Most people simply take an intro course, don't talk to anyone then complain it's hard.
French is hard even for people who speak it on a daily basis.
I am neither a linguist nor an historian (so google it and find reliable sources for more information) but AFAIK, it was designed and reformed to be an "elitist" language (for political reasons), there are gazillions of special cases that do not make any sense (both in the written and spoken language) and there are heated debates about what should be kept or removed (in the perspective of teaching it for instance).
