I see your point, but the difference lies in Hell. There are businesses/restaurants in the bathrooms of which one will find a sign posted near the sink reminding employees to wash their hands.
I currently work at an early childhood education center in its After-Care program, and back when I was doing my online training I remember being instructed about how staff and children must wash their hands (e.g. scrubbing your hands for 20 seconds). I don’t think most people are obsessive over these hand-washing exhortations or suffer anxiety on account of them.
But then again there is not the threat of Hell behind them.
On the other hand, mortal sin – even one – means eternal damnation should one die in that state. And can anybody think of anything worse than eternal damnation?
The thing with the manuals is that it reveals to us all kinds of things that are considered sins, and the way the scrupulous mind processes the identification of sins goes something like this:
Look, lots of (mortal) sins. Lots of mortal sins because it is so easy to sin. It is so easy to sin because there are so many opportunities to sin. With so much ease and so many opportunities it is likely that I have sinned/will sin mortally. Therefore, etc… insert here all kinds of fears about the present state of one’s soul and all kinds of perceived precautions one must take to avoid sinning, e.g. avoiding people).
Basically its a feeling of being overwhelmed by what seems like the immense difficulty of avoiding mortal sin and the temptation to regard mortal sin as inevitable, hence temptation to despair, etc.