Easy.
Each one of us, and especially politicos, will first and foremost look out for our self interests. So when politicians get power the temptation to use it to advance their self interests is very strong. It’s really not surprising at all if you understand human nature.
The answer is to give them the bare minimum amount of power and money to do their job and to watch them like a hawk.
The founders provided the bare amount of power by limiting government legislative powers but the people neglected to fulfill their part.
"The condition upon which God hath given liberty to man is eternal vigilance; which condition if he break, servitude is at once the consequence of his crime, and the punishment of his guilt."