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.
I could see something like - someone is the best of the best, gets selected for a very prestigious Secret Service posting, then nothing happens and they just have to pick up dry cleaning and watch the fanciest and most pretentious people in the world attend cocktail parties for several years, and eventually they end up visiting sex workers and drinking on duty and things like that