It's because it is all about greed.
Retail banks are necessary, yes. But they are in a priviliged position. They have access to/control of the money supply and have that because the government trusts them to distribute money into the economy.
On the back of retail banking, they can get a nice cut and lead a nice happy life handing out mortgages and business loans. Not a stupid billionaire life, but a comfortable one.
Then sometime before the GFC, the retail banks who did the above started transforming themselves more into investment banks. They did this because the greedy buggers at the top decided they wanted more money and bigger bonuses than just their happy comfortable life cut. Investment banks take bigger risks and therefore earn more money at risk of being totalled if they get things wrong or if there is a big financial crash.
Here's the thing though. When a retail bank that is conducting significant investment operations goes bust, it not only takes down its investment arm. It takes down its retail arm, or in other words, the public and businesses in general. This makes a lot of people (voters and small businesses) really mad and as well can be catastrophic for the economy. So the government has to step in to bail them out.
So basically they are gambling with public backed money. If they win they walk away with massive bonuses from their investment operations. If they lose the government bails them out with taxpayers money.
There was an act in the US that separates retail and investment banking called Glass Steagall. This effectively separated out banks so they couldn't conduct both types of operations. Unfortunately the financial institutions put massive pressure on to undo this, because they want the funding and backstop that retail banks provide so they can get on with their gambling and make millions. Until they someday don't and the poor bugger in the street has to cough up for it.
If we do not stick to separating out retail and investment banking and making them as isolated as possible, then sooner or later exactly the same thing will happen again as what happened in the GFC. We do not appear to be learning from history.
This is why I don't like banking and bankers in general.
Finally, a quote from Henry Ford :
"It is well enough that people of the nation do not understand our banking and monetary system, for if they did, I believe there would be a revolution before tomorrow morning."