Why should a company not get 100% of the shares upfront?
If a company were to take 100% of the shares upfront, the risk of over-delivery is approximately 50%: in scenarios where the average share price turns out to be greater than today's, the company took too many shares on day 1 and needs to return these shares, or pay additional cash. That has various implications including on taxes, collateral requirements by the bank, and liquidity management at the copmany.