Two additional points: consent and "health benefits"
Adults understand the concept of a medical procedure, the doctor can administer an effective amount of anesthesia or even fully sedate, and opiate pain medication can be prescribed during recovery.
Additionally, for an adult the penis is fully grown, and the man can express his preferences like high vs low cut, keep frenulum, etc. For an infant, the doctor can't know how much the penis will eventually grow and just makes a best guess when cutting.
The adult foreskin is also detached. Normally from birth to puberty, the foreskin is fused to the glans/head of the penis in much the same way as a nail on your finger. During circumcision, a probe is used to break and separate the skin. Imagine the pain from pushing tweezers under the nail of your thumb all the way down - now imagine that pain on your penis.
A baby boy doesn't know why he's being strapped down onto a cold board (called a circumstraint). And regular anesthesia methods can't be used on newborns - at best only a local pain killer is given - only for him to feel the most innervated piece of skin on his body being cut off. Nurses often tell parents that "he slept right thru it", when in reality the baby went into shock from pain.
Infant circumcision is genital mutilation.
Boys being cut in the US has it's roots in sexual control. Regarding masturbation, John Harvey Kellogg wrote in 1888:
A remedy which is almost always successful in small boys is circumcision, especially when there is any degree of phimosis. The operation should be performed without administering an anaesthetic, as the brief pain attending the operation will have a salutary effect upon the mind, especially if it be connected with the idea of punishment, as it may well be in some cases.
The foreskin is not just "a small piece of skin". When erect, the average penis has a length of 5.2 inches and a circumference (girth) of 5 inches.
Most circumcision procedures will cut 1 to 2 inches behind the tip of the penis. Let's do the math:
1.5x2=3 length of skin removed (average of 1.5") times 2 (the foreskin is a doubled-layered fold)
3x5=15 length times circumference
In an adult, that's at least 15 square inches of densely innervated skin that has been removed.
Back in 1992 the Southern Pacific Railroad offered to sell the entire coast route between LA and San Francisco, plus upgrade it for fast-train operations for $360 million dollars. Even adjusted for inflation, that's only $638.5 million. With 110mph track, the entire 483 mile trip could be completed in less than six hours. (And it would have been built by people who knew how to actually run a railroad.)