It's actually Christmas & exchange rates mass. If we go by international exchange rates of 2009 to 2019 then EndGame will have crossed Avatar by now.
Also the summer competition which is much lesser than Christmas.
And it'll be difficult for Avatar 2 to cross Avatar as of you look at Avatar then only thing that made it highest grosser was 3D & Technology, other then that it had nill effect on pop culture, media, and it's not even a classic movie which carries on the goodwill to its sequel.
Curiosity factor of being sequel to highest grosser may give it big opening but if story is generic like first one then it won't get the required legs to touch the heights of first one.