If they did not play accounting games they could pay actors based on movie revenue without increasing upfront costs. However, major studios are setup as cost plus contracts where they get upside on successes and still make money even if nobody sees the movie.
Also, if you look at the actual costs it's not hoard to find advertizing more than 50% of the budget for mildly expensive films. Because advertizing is effectively a huge fixed cost they are incentive to make expensive movies with big stars and or explosions that they can leverage to make their advertizing less expensive.
EX: Jurassic park and Avatar could show Dinosaurs and so did not need big name talent to get people into seats.
Also, if you look at the actual costs it's not hoard to find advertizing more than 50% of the budget for mildly expensive films. Because advertizing is effectively a huge fixed cost they are incentive to make expensive movies with big stars and or explosions that they can leverage to make their advertizing less expensive.
EX: Jurassic park and Avatar could show Dinosaurs and so did not need big name talent to get people into seats.