Streep is an exception (and good for her if she can still pull in that kind of money.) Most actors don't pull in anything like that amount of money, and even those that are able to pull in six digits or, occasionally, seven, digits per movie do so usually knowing they have a shelf life, and that Hollywood will discard them when they get into their 30s. At that point, many know they'll be difficult to hire in any other professions, as they just devoted much of their lives to a single profession, and have no skills outside of that, and have fame as an added handicap.
20 million, incidentally, is dirt cheap for a modern movie (to put it into perspective, the pilot episode of the 2000 TV series Dark Angel cost that much), and the right star can be the difference between a $60-250M movie (which is more the ballpark) either making a loss, or making an outrageous profit. The ticket price, which seems to have held steady at around $10 per adult for the last 20 years now, is what the market has determined is what people will pay, so that's not going to come down if studios were to cut actors salaries. So... why complain about this, specifically? If they're the ones making the movies profitable, and if the money's there, why not let them have a cut?