Because nobody on the better paid fields would take such a contract. You could try it but you would most likely lose in the market place of jobs by all the best employees going somewhere else.
As to why having a contract that pays you for the standard work week always I think people just like the security of getting roughly the same amount of money each month (or biweekly or however often you get paid). Especially once you have some money so you have mortgages, loans and credit cards to manage on top of your monthly bills.
Also the hours beyond the normal work week are overtime and come with multipliers for your effective hourly rate so companies try to do their best to arrange things so that minimal amount of overtime is actually done.
The point of salaries was to pay employees by the week instead of by the hour. My first job had a weekly timecard that consisted of a checkbox: "Did you work this week?"
Now, every job I have requires me to account for all my hours, so it seems like the only reason I am not a wage employee is to avoid paying me 150% base rate for hours worked beyond the first 40 in a calendar week.
As to why having a contract that pays you for the standard work week always I think people just like the security of getting roughly the same amount of money each month (or biweekly or however often you get paid). Especially once you have some money so you have mortgages, loans and credit cards to manage on top of your monthly bills.
Also the hours beyond the normal work week are overtime and come with multipliers for your effective hourly rate so companies try to do their best to arrange things so that minimal amount of overtime is actually done.