mainly because the two agencies should have never been combined to begin with. There should have been a separate 'lock box' account set up for DI and rules should have been set up long ago to never allow other agencies to grab funds from either DI or SS with an I.O.U. piece of paper. As SS payments come in all of it should go directly into SS account and have Congress appropriate funds in the annual budget for DI as a separate entity. Another way to collect separate funding for DI is to, just like was done with Medicare, have a separate deduction made from employees' paychecks specifically for DI.
The whole reasoning behind having DI be separated from SS, I believe, is since 2007 as unemployment benefits ran out for millions of people, many of them were automatically filing for DI benefits (and approved under the 'mental duress/depression' rules which has spiked that number outrageously and there isn't enough money to fund both anymore. There needs to be more/better scrutiny for claims....which is what Reagan did in 1983 to the SS/DI system and needs to be done again. Many, including myself, were removed from DI and had to have our claims re-evaluated. He saved the system billions by finding thousands of fraud claims because the system prior to then was very lax regarding approvals.
This one tip will help you sleep better tonight
A few seconds are all you need to get a better night's rest.