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

A rank does not make a leader. Under General Mille, the most important thing in his life is how to properly address the opposite sex and queers and notifying China before, should we decide to attack.
This is not the armed Forces I was in. The Iranians continue to tweak our nose, Saudi Arabia laughs at us, Hezbollah considers the U S A military forces a joke. And we are discussing SP5? When agreed we going to wake up?

The Army being the largest force in the military doesnt really have an abundance of leadership roles for everyone advancing to E4. So when SP4 promotion comes and that soldier proves very effective his unit can recommend Corporal, get him to the Sgt board.
The USMC is a much smaller force. All promotions are based on performance or better known as fitness reports.

I wish the technical spec ranks had persisted personally. I was a spec4 for far too long. My skills advanced but my padid not. I was toying with ocs due to completion of my degree. But life circumstances and injuries kept me at E4 eternally I seemed.

To answer this question, yes and no, a SP4 has some leadership duties when that person is put in that roll, a Coporal steps right up when the occasion arises, both can lead, mentor, give advice, and look after their squad, and most of all be ready to carry out assignments

Simple. Because not all E-4's can be corporals. An E-4 is promoted to corporal when a Specalist doesn't have the points to be promoted to Sergeant, but an NCO is need in that particular section.

Specialists were supposed to be the enlisted 'technicians,' with emphasis on working in their skill area instead of taking time away for leadership responsibilities. Didn't quite work out that way. Most of the SP6 and above that I ever saw were in the medical fields. I made it to SP5 in EOD back in the mid-70s, but anything above that in my field carried Staff Sgt rank.

I am not sure what you are talking about. There has not been a SP4 rank in the Army since before I joined in 1991. By your picture I will assume that you are from the Vietnam era and the SP ranks were very prevalent. The current Specialist rank was adopted in 1985 and only the old timers still call them SP4. And while a specialist in the position of team leader is the leader of that team he/she is not the same as a Corporal. They are not considered a Noncomissioned Officer they are still junior enlisted soldiers. When I was in service in order to pin Corporal stripes you had to do a few things. Be in the NCO position for a minimum of 90 days, be promotable and not have any negative actions pending. i.e. no bars or pt failure or weight control issues. Even then it was very rare to get laterally promoted to Corporal.

I will say that I do think that they should bring back the senior specialist ranks. There are some people that are excellent at their jobs, yet have no business leading Soldiers. And while we are talking about it once a Soldier makes the rank of Specialist they should get rid of the retention control points and allow them the ability to retire after 20-30 years.

The SPEC ranks needs to come back. They should have never been 86. The ranks was sent South one by one. I was a SPEC 5 when the Army converted me to SGT around 1985. When it came to lead, I lead. But was not a born leader. A Specialist rank was specifically to his / her techeninal MOS. A Hard Striper was a generic leader to all MOSes.

The Specialist rank is career specific. Not all jobs in the military are leadership roles. Squad leader roles are corrals but clerical may not involve other troops

My thoughts on this are, most E-4's are not ready , and maybe will never be ready to make CPL, but are technically adapt enough to get the job done without supervision.