core language - Why can't a string be formed by head String?

Since everything is an expression in Mathematica, why must a string object be formed by "abc" but not by a String[abc] expression?

You can look at a string's head by:

Head["abc"]

String

But you can not produce the same string by String

String[abc]

which, from my point of view, seems inconsistent with the principle that Everything Is an Expression.

However, I noticed that the basic Symbol object, on the other hand, can be formed by something like Symbol["a"].

The same question goes for four number objects (Integer, Real, Rational, and Complex). You can't say an integer 1 by something like Integer[1], can you?

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Edit:Rational and Complex can be produced by their respective heads. So The question is valid only for String and two number objects, i.e. Integer and Real.

Answer

String and Integer are what I termed "implicit heads" while writing:

Rather than being part of the standard expression itself, at least as I understand it, these implicit heads instead serve the purpose of providing a "type" for pattern matching. (With a pattern _String, _Integer, etc.) The atomic expressions themselves are stored in a low-level format and handled transparently behind the scenes.

Of the heads you list Rational and Complex are exceptions as these are a kind of hybrid head: you can use them to enter data:

{Complex[1, 2], Rational[5, 8]}

{1 + 2 I, 5/8}

Critically you can also match patterns within these heads:

{1 + 2 I, 5/8} /. {Complex[a_, b_] :> foo[a, b], Rational[n_, m_] :> bar[n, m]}

{foo[1, 2], bar[5, 8]}

Nevertheless these expressions are considered "atomic" and they cannot be manipulated other ways that apply to standard expressions:

AtomQ /@ {1 + 2 I, 5/8}
foo @@@ {1 + 2 I, 5/8}

{True, True}

{1 + 2 I, 5/8}