It was primarily a political reason; Wilson had made it clear that American forces would serve under an American general, and forbade Pershing to allow American units to be detached. In the end, pershing relented, with some units such as the Harlem Hellfighters fighting alongside the French, and even some British units.

/u/WinglessFlutters hints on this with his first point, about this ceding control of American units to the French and British, cutting Pershing and the AEF out. However, I will take issue with his other points. Logistically, the Americans were entirely reliant upon the French and British for artillery, aircraft and tanks. American troops were armed largely with British and French weapons as well.

I also should emphasize that it had nothing to do with Europeans being 'free' with their men's lives. In fact, Pershing was critical of Franco-British methods as too 'cautious', putting too much emphasis on firepower, whereas he wanted American troops to fight 'open warfare'. While there would be some 'open warfare' in the Hundred Days, American forces were largely bogged down in the Argonne, where an insistence on 'open warfare' tactics better suited to 1914-16 than 1916-18 produced some of the worst casualties American forces ever suffered since the ACW. Here, allowing American units to fight alongside Franco-British forces before hand, instead of jealously guarding them until summer 1918, could have disseminated the skills and methods to avoid this bloodshed.

If American soldiers were divided up, there wouldn't be a cohesive US force in Europe. Rather than having control of logistics, supply, troop rotations, as well as tactical control of those units, a divided force would cede all of that to British and French commanders.

There could be logistics issues, with units issues improper weapons, ammunition or equipment. If those dispersed units were to be supplied by a US logistics train, dispersion greatly magnifies the difficulties. If they were to be supplied by local logistics, there could be a conflict of interest between the priorities of local commanders and US forces.

Ceding tactical control of units means that a different commander is now making judgement calls of how much the life of a soldier from your country is worth. WWI was rather bloody, and perhaps there was the opinion than European commanders were too willing to spend lives.

Politically, not having an American sector of the conflict reduces prestige. Any success can be assumed by the higher command, but any failures can be attributed to that US unit. With American battlespace success or failure is far more objective.

This isn't anything historical, these are pragmatic concerns which are clearly still relevant today. Specific historical questions I have are: To what extent were British, French and American small arms, artillery, and vehicles similar and different in WW1? Was there an Allied effort to simplify logistics, similar to the modern NATO round?

What was the American opinion about the casualties in Europe? Were casualties perceived to be the fault of Allied commanders? How willing was the United States to accept casualties?

Was National prestige a great concern for the United States during WW1? Was the US perceived as a lesser nation than the traditional European powers? What positive and negative publicity did the American expeditionary force in Europe receive?