Because allowing new public legal tender to be created on security of fictitious capital such as speculative land values and deposits of credit created by other banks is accounting fraud, transfers wealth from the poor to the rich, creates speculative bubbles in financial asset markets, promotes disinvestment in the real economy, decreases demand for labor, inflates the price of land relative to wages for unsupervised labor, and worsens inequality.
The only reason inflated valuations based on speculative hype, i.e. your 'fictitious capital', are able to redistribute wealth from the productive economy to rent-seeking interests is that parties taking irresponsible risks are bailed out by government programs that socialize losses. These programs are sold to the public as making the market safer for consumers:
Real and Useful: people can use the money as a store of value, medium of exchange, and a unit of account - and enough people believe in it.