“Does Hollywood truly have mastery in making high-concept movies such as Titanic, Avatar and super-hero films? Could that be the main reason?”
Only if by “truly have mastery in making” you mean in making money.
They have the most investment, name recognition, strategic marketing skills (and developed relationships with theater chains, etc) at making money from such movies, and are almost the only ones investing that much in making and marketing those movies.
I wouldn’t leap from that to the words “truly have mastery in making”, because those words to me seem to be about making great films, and I’m not particularly impressed by what they’re making, especially not the writing and choreography, which seem to me to be pretty mediocre in many ways. The formulas they used are designed to make them money, not to make films that are good in other ways than box-office sales.