Because having only one entity that lends money is a bad idea...
Certainly the line of reasoning that the bailouts could be skipped altogether in an idealised system is very persuasive as a propaganda line for people proposing terrible policies like "what if it was impossible for people or businesses to borrow money without the approval of the state board for borrowing money" (or "what if ordinary depositors were subject to the same risks as capitalist investors") and arguments like the OP's which conflate the concept of maturity mismatch and insolvency are very persuasive to people that don't know the difference. But personally, I prefer to analyse how things work rather than compare their respective propaganda values