For the same reason as heroines must retire at 25 or so. Their luck runs out.
You get signed up because you are someone’s daughter, or because you’re pretty, or because you’ve had a nose job.
9 chances out of 10, your first film flops. But if you succeed, you sign up five more. 9 chances out of 10, your second film – if there is one – makes money. But that doesn’t matter because you are already booked for five films, three of which may get made.
Now, what are the chances that none of the five will have a positive return on investment? Small.
Besides, your producers’ PR are working overtime. Indeed, even something as trifle as your appearance in someone’s birthday party is reported.
Anyway, you get the sequence: Hit Flop Flop Flop Average Flop Hit FFFFAAHHF. Somewhere you get a good run, when you make your nest egg, but mostly it’s bad.
When it starts going FFFFFF for too long, you have no option but to fly. The fundamentals have given away.
I suppose its the same with geniuses, except (a) they can stay afloat longer – score so-so – by simply mimicking others. And one big hit is often enough to see you through much of your career. All the same, you can’t tempt luck too much. You must retire at 40 or, more likely than not, you’ll be retired at 41.