Australia and India LO batting side suffers from the same problems, don't they? Insanely strong top 3 who take up most of the balls everytime they play and a chicken and egg situation that it causes with the middle order not getting enough opportunities and time to show how good they are/can get to be.
I think a lot of teams are having this issue more in T20 cricket, because:
1. 4-6 (particularly #5 and #6) are really hard places to bat in T20 cricket in the first place - often no time to get set at all, no field restrictions, coming in towards the end of close chases etc
2. Because batting top 3 gives you a lot more ability to influence the game, most talented batsmen end up batting there domestically for the good of their teams, and as such don't develop the skills unique to middle order T20 batting
3. Given #1, most the good domestic T20 middle order bats end up with less impressive records even when they're contributing well, and given #2 these players tend not to be highly rated in the first place - so they don't end up selected
As such, you end up with a whole bunch of players trying to learn the role on the job in international cricket after opening or batting three at lower levels. And as with #1, even if they do a good job at it they'll be under pressure because they won't have great looking numbers.
You're right that India and Australia seem to have this problem
even in ODIs, but I think part of that is balance issues trying to also five bowlers and a keeper in there.