In most companies I've worked for the sick note requirement was 100% due to a few bad actors. We had to ask for one everytime because they wanted to avoid potential lawsuits if we only asked due to a pattern. 99% of people were doing the right thing and this policy just inconvenienced them. Part of the issue is employers making rules based on the 1% of bad faith situations. The amount of time management and HR spent running after doctors notes would have easily covered the cost of that 1% of shitty employees if notes weren't required.