Well, technically, The Force Awakens actually has made more in its first run than either Titanic or Avatar -- Titanic made $1.84B in its first run and Avatar made $2.004B. Both were re-released to get to their current totals. TFA is currently at about $2.062B and I wouldn't be surprised to see a re-release in the future (probably right before Ep VIII) which would get it past Titanic's overall total.
It's also worth noting that, while the higher prices/inflation is certainly a major factor, The Force Awakens was released in a time of a relatively strong dollar which has depressed overseas box offices numbers.
Edit: Not that any of that really matters. TFA isn't the most "successful" movie in history on basically any meaningful metric, but why would that matter? It certainly was a massive success and over $900M in profit from just the box office alone (not to mention the impending home media sales, TV rights, and ongoing merch sales) is pretty amazing.
And, besides the revenue, people actually y'know tended to like the movie. Which is kinda a good thing too.