I imagine it’s just incredibly difficult to take your job super seriously when any given thing only happens like once every 5-10 years, and probably not to you. I imagine most secret service people who are doing security spend 100% of their careers just standing around and then retire with nothing having happened.
At one point, US embassy security details had this problem, and what they settled on was rotating active-duty combat troops in straight from the field so they were super alert. After about 6 months they would start to relax, and they would rotate them out and have fresh people.
I won’t claim to know what the answer is for the SS but clearly there are some issues with the way they’re doing it.