So why is is assets = liabilities + owners equity?
Look at a few steps of this example.
I am a business and I borrow $100. Now I have an asset (the $100), and a liability (the $100 is owe).
so the equation is
cash + stuff = liabilities + equity
$100 + $0 = $100 + $0
Now I use the assets to buy $100 worth of products to sell. I just changed the form of the assets. So the result is this:
cash + stuff = liabilities + equity
$0 $100 = $100 + $0
I have a good day, and I sell everything for $250. The result is that now the equation looks like this.
cash + stuff = liabilities + equity
$250 + $0 = $100 + $150
I can now use the $250 to buy more stuff or pay back what I owe and then use the equity to buy more stuff. and the cycle continues.
It does get more complex when you are dealing with taxes, depreciation, selling expenses....
The equation has to be structured this way because you can't look at the pile of cash and think you are profitable.
Notice then when banks talk about assets and liabilities the amount deposited into the customer accounts are liabilities, the loans are assets. It gets confusing very fast.