Why does the US Army persist with the rank of SP4?

I was a SP-4, SP-5 1961-1963. After active duty I ended as DAC personnel specialist, highest grade GS-13. The training I received on active duty required 8 months full time classroom equivalent. There really wasn't time for normal military skills training. I was on a 3-year enlistment. For the Army to get "its money back", have a rational investment in me they paid me for learning and exercising my skill. I was very good at my job and I believe the Army got its money back. However there was no way I was qualified to move into a combat arms unit. I would have gotten people killed needlessly. Another point not mentioned. During my DAC years I saw an increasing use of DACs in combat zones as advisors, technical specialist and trainers. There is an absolute need for technical specialists along side combat arms personnel. Whether the specialist is a DAC or an SP-5/6/7/8/9 needs careful evaluation by the Army. (think insurance, training, equiping, responsibility and chain of Command)

No it's not, I was SP4 in 1989 and 1990. I was a team leader and acting Squad leader of 4 teams when the Squad Sargent was hospitalized. I was scheduled for PLDC school, but Justcause popped up and was deployed to it.

military and non military have ongoing deeds and activities regarding hierarchies and ranks and grades ranging fron sensical to nonsensical

I got out of the USMC in 1958, and joined the army guard as a Spec 4 in my home town. (117th AIB). When we went to two weeks of summer camp, the first thing I noticed that the Army was not on marching they could march in a straight line) but other that they were lost. (The master sergeant call me out of ranks one day while we were marching down the street, and said march them. I said I can’t march them as there some higher ranks them I am in the formation, he said march them. I gave them a command of “Left Oblique” the formation was all over the street. I called ‘Halt” fall in. the master sergeant asked what kind of command was that. So I went through some of the drill commands that we used in the USMC. That was the last time I was asked to march the formation.
From what I was of the army my short time in the Guard was that all ranks needed more training in leadership, and the job each man done in their unit. (We were taught in the USMC the job of each man in the fire team and if a PVT or any other rate could take command of the unit if he was he was highest ranking person left to command.
I had a E-6 working for me in the navy who didn’t want to go any higher (I would put him in for E-7 he would take the test, he said he marked number four on all the question and turned it in). He was an outstanding E-6 as he knew his job, trained his people well and set an example for the people he was in charge of, but he didn’t want to go any higher than E-6.
Maybe the Army need to rethink their training of their people.

The specialist grade depends on what your MOS is.

Supply, in a sense by essence of the job description, spec4 as clerk is a good thing rather than corporal, however for command staff and discipline within logistics need top notch NCO's like everywhere in the military.

My son is infantry, if the supply chains lack any necessary discipline, he struggles and we lose the infantry, well guess supply is going to have to be the front line

The thing that confuses me when I see questions like this, and all these replies, is this is exactly what the Warrant Officer corps is doctrinally designed to be; technical specialists and advisors almost entirely without leadership or command requirements. Many of the Warrant specialties do require E5/E6 rank for selection. However, the Army is considering removing *any* prior service requirements for some specialties - historically Aviation Warrants have serve as an example. If the selection process needs to be revised further to provide for that, we should do that.

So that you can pay soldiers more, without having to make them NCO's.

I am out of my element to a point. I served in the Navy. We had NEC's. As we advanced we took leadership courses along with our MOS or NEC. Navy courses and navywide exams. You needed permisssion from chain of command to take courses and the Navywide exams. You would not be advanced without thse steps. Advancement meant more responsibilities for the rank and pay. You could be the best technician in the world but the leadership part comes with that. Onboard ship every manand woman needs to have the others six. That is why we were trained and cross trained.

It seems to me that the military has lost sight of what the specialist rank is for. Transportation for instance. Specialists drive the trucks. My most was 64B20. I drove a 5 ton S&P delivering stuff to all the military bases in most of West Germany, including Air Force bases. That required a specialist rank, not a Sgt. rank. If you want to be a platoon leader go to NCO school.