They are expected to be, and they diet and exercise to be that way.
If your question is: Why are most actresses skinny? Society requires them to be and the camera is not forgiving to plumper figures, especially for women. If your question is: Why do most actresses look skinny? They are actually skinny accept if you are looking at pictures, then you can bet they have been Photoshopped. If your question is: How are most actresses skinny? They can afford to be. They have personal chiefs, trainers, and dietitians at their disposal. If that doesn't cut it they have doctors who will fix them with drugs or surgery.
The cast of Skinny Gets a Goat - 1917 includes: Dee Lampton as Schemer Skinny
The cast of Skinny Routs a Robber - 1917 includes: Dee Lampton as Schemer Skinny
The cast of Get Skinny - 2002 includes: Carol Ann Leif
Because all modern actresses have to look skinny.
The cast of Skinny Love - 2013 includes: Nina Diedrich
Because all modern actresses have to look skinny.
The cast of The Real Skinny - 2012 includes: Sarah Engle as Patient
because peolpe say they are fat.
The cast of Skinny and Cat - 2014 includes: Rachel Weisz as Margaret Bourke White
The cast of Skinny Jeans - 2012 includes: Kat Foster Brian Gattas Laura Kachergus
The cast of My Life as a Skinny Puppy - 2010 includes: Nathan Andrew as Jim Jones