why do jobs expect you to bring a doctor's note when you called in sick the day before?

Some jobs expect a doctor's note because some employees call in "sick" when they are actually standing in line for concert tickets, or skiing, or hungover, or any number of other things.

too many not showing up for work

the last job i had required a doctor's note, people lie and will do it to get off out going to work. usually they want to go to a concert or they are hungover.

Because they don't believe on them

To prove you were sick.

In my case it's because the tight ar*e that I work for is looking for any excuse whatsoever not to pay you your sick leave entitlement.

Your predisessors were liars

Not mine.

Because it was on their time and you work for them, not the other way around.

To prove you really were sick enough not to come to work.