One thing to realize is that WWII was the largest war in human history, both in terms of manpower deployed and resources consumed.
After the war the U.S. cut back its military to a huge degree. All those millions of men who were drafted were sent home. All those war machines we built (ships, planes, tanks, etc) weren't needed for the much smaller peacetime military. We were literally pushing planes off of aircraft carries, bulldozing fighter planes on the runways, and crushing brand new jeeps, because we just plain simply had too much stuff to use and didn't want the expense to maintain or store it.
Our Navy mothballed or scrapped many of our ships because we didn't need them and didn't have the manpower to crew them. The captured enemy ships were even less useful (in most cases) as they were non-standard to the rest of our equipment and weren't worth the effort to maintain them (much less crew them). We didn't have enough manpower (or need) for our own ships, so why would we want the enemy's?
We did examine things like German submarines for any technical secrets they could reveal but, generally, once studied they were destroyed. Also, post war treaties called for the destruction of specific enemy equipment, such as German U Boats. (The reason we didn't destroy the U 505 was we argued it was captured in battle, not surrendered, and didn't fall under the treaty).
Also, with very few exceptions, the U.S. ships were better designed and more functional than Japanese, German, or Italian ships.
