IPO Bidding Question

I understand the basics of how an IPO is underwritten using the book building method, but I'm trying to figure out what prevents bidding investors from all submitting bids at the floor price...?

My understanding is as follows:

1. An investment bank underwriting an IPO builds a book by providing potential investors with a floor price and a ceiling price between which they can submit bids. 

2. The bank then collects all the bids at different price levels and aggregates demand to arrive at the final share price, also known as the cut-off price.

3. Shares are allocated to accepted bidders at the cut-off price. Investors who bid in excess of the cut-off price are refunded their excess payment and investors who bid below the cut-off price are asked to pay up.

My question: Why don't all bidders submit their bids at the floor price with the idea that the cut-off price will be lower and they can all get allocated shares at a lower price?

Any insight is appreciated. Just trying to learn more about the process. Thank you!