There's another big reason: both Democrats and Republicans have been borrowing Social Security funds and using them for other purposes for years, in order to buy votes for themselves.
Actually - the answer is worse.
The Democrats are Santa Claus - seeming to promise wonderful presents to everyone.
The Republicans have tried to be the stern doctor forcing Americans to take the nasty-tasting medicine (think Republican revolution 1995 - 1996). The Gingrich revolution was turned topsy-turvey with the media portraying Newt as the “Gingrich who Stole Christmas.”
Then - and here is where it gets VERY POINTED - the majority of the public turned on the doctor and showed that they wanted Santa Claus.
The Republicans decided that it was more fun to get re-elected, so they might as well join the Santa Claus party - only they would play Santa-Claus-lite.
So yes - the Republicans got into the spending - but it is because the public gets the representation that they deserve - and most are so stupid - they deserve stupid government - and that is what we got!
Republicans with honesty and integrity tend to get tossed out in many states (not all - but in the purple states or blue states) - so they gave up integrity and adopted the big spending habits of Democrats. But the real fault lies with the public.