(Also Is this just recency bias at play? Were the Spanish American War and WWI that big of a deal at the time? I mean, I understand the scope of the wars, but were they that much more serious than the other wars I mentioned?)

There is a significant amount of recency bias and because we are used to the organized military ranks of the modern military, whereas we must look at figures like Pershing and Dewey in the context of a time when the modern US military structure didn't exist.

Keep in mind that the US Army was tiny before World War I - from the DOD Statistics of 1997 pdf, you can see that the US Army in 1916 had a grand total of 108,399 personnel. By the end of World War I, the US Army had grown to 2,395,742.

When a military grows, so do the number of units: if the US Army in 1916 only had X number of divisions, then in 1918 they had a magnitude more number of divisions and corresponding layers of organization above (corps which then fall under armies).

Not only was this the largest military the US had ever had, it was also the first to be deployed overseas on such a scale. That brought it major organizational changes.

For one, keep in mind that in peace, the US Army was divided between the Regular Army (the permanent active duty component made up of professionals), Army Reserves, National Guard. The National Army (later called Army of the United States in WW2) was the Army raised for war and consisted of volunteers and conscripts.

As thus, professional soldiers in the Regular Army could hold temporary wartime ranks in the National Army - thus you had two-star generals before the war get promoted to 4-star general positions (due to the need for them with the expanded military) that reverted back to their temporary rank after the war.

Thus, Pershing, who was going to lead the American Expeditionary Force - the largest force of US military personnel to date - was not only given a four-star rank (the highest at the time), but due to the significance of his position, he was given the honor of having the distinct title of being a General of the Armies (he was literally general in charge of multiple Armies with Armies being an organizational unit)

(Dewey was created an Admiral of the Navy to honor his victory at Manila Bay at a time when the Navy didn't have many admirals of high ranks, so it was largely honorific)

Don't discount politics either: during World War II, the five-star rank was created in part because you had people like Eisenhower, who commanded multiple four-star generals AND was Supreme Allied Commander and who had subordinates in allied militaries that had five-star equivalents (like Field Marshal) - thus it brought them to the same high rank.

edit: I want to talk about a common misconception. At the 3-star and 4-star general levels, one does not simply get it by getting promoted - one is given those ranks based on the job you have.

For instance, in the Navy, the Chief of Naval Operations is always a 4-star Admiral. You can be a 2-star admiral and get selected for CNO and automatically become a 4-star Admiral. Once you leave the job of CNO, you revert to your 2-star rank - unless you pick up another 4-star command (typically rare, and most just retire to retire as a 4-star).

US Title 10 actually governs how many generals/admirals each branch can have.

There are actually exceptions to this total - for instance, the person in command of a combatant command like Pacific Command or Central Command is automatically a 4-star. A great example is the Marine Corps typically only has two 4-stars: commandant and assistant commandant. However, General Joseph Dunford is the current Chairman of the Joint Chiefs of Staff, and General Thomas Waldhauser is commander of US Africa Command, so the Marines actually have 4 four-star generals right now.

So for much of American military history, with our tiny military forces, we often didn't have any admirals or generals even above a 2-star rank. Pershing and Dewey were given their ranks both as an honor and to illustrate that t hey were in command of much larger forces than the US traditionally had.

It is important to remember that according to American Military Tradition, no general can outrank Washington. At the time of the initial formation of the Continental Army, there was no rank higher than major general outside of Washington's position as Commander-in-Chief.

This made sense. The United States Army was never larger than a few divisions, with a major general traditionally commanding a division. This is until the Mexican War, when Winfield Scott was named a Lieutenant General.

At the outbreak of the Civil War, even with much larger armies and army corps, in the United States, the tradition remained to name either regular or brevet ranks of major general. As to Grant, this became a problem. In the winter of 1863, when Lincoln wanted to name Grant as Commander-in-Chief of all Union armies, he would not have been able to without promoting him. This is because as Grant was a major general, there were several Union officers who were senior to him who would not have legally been able to give orders to. To solve this, Grant was nominated for and given the rank of Lieutenant General. After the Civil War, the commander of the United States Army was traditionally a Lieutenant General.

The Confederacy did not conform to the Union rank system, instead almost immediately adopting the ranks of brigadier (brigades), major (divisions), and full general (armies), and adopting the rank of lieutenant general in the fall of 1862 for corps commanders.

So the rank of the General of the Armies did not even exist until World War I. This also represents the size of the armies deployed.

As to your other point, Patton only commanded up to 3rd U.S. Army (if you do not count U.S. FUSAG). Most U.S. Army commanders during World War II were lieutenant generals. John Paul Jones never commanded more than a squadron (likewise for Barry).