I like to think of governments as an agency from the platonic realms ruled by incorruptible selfless beings with term limits who look out for all life on earth.
Unfortunately, governments need money. There are only two ways to get it. Taxes and Inflation (increase in money supply). The banks won't honor money that loses value (because of increasing supply) unless they get a share of the newly created money. Without governments sharing newly created money with the banks, the banks won't just honor money as a payment for debt.
Since weapons manufacturers need the banks to honor money, governments can't force the banks to accept money that loses value, because governments need weapons to create credible threats to the banks to honor money in the first place.
As long as banks get newly created money for cheap, they will offer it to new politicians they like so that once they get elected, they will help the banks out.
I like to think of lobbying as a secret meeting around a round table at night. Or a late night knock on the door at a senator's house.
Most lobbying needn't be done. It happens without being said. Powerful people are aware of what they need, and align themselves so that they can empower each other while benefiting themselves.