Why are private and hard money necessary?

[Tyco Bronye]

I'm still in education mode, I haven't purchased a house besides my own. I'm not fully understanding why private and hard money is so necessary. It seems that if you were solely in the business of flipping you would soon be able to fund your own projects in a short time. Also it seems that banks loan fairly easily on 4 properties so you could have 4 flips going at a time at a much cheaper bank rates.

Let's say I'm starting out with my first flip. A house in need of rehab is 60k. I need 15k for down payment and 30k for rehab I spend that 45k. Then when I sell I get all that money back plus 15k in profit so now I have 60k in the bank.

I do the same thing over and over now I have 75k in the bank, then 90k, 105k etc. Lets say you did a house per month and made 160k a year. It seems like even lending from banks would be unnecessary in a relatively short time (under 2 years)
At that point you could just buy everything with your own cash and eliminate lending entirely.

What am I missing?