Why can't I have a numeric value as the ID of an element?
That's just what the spec says.
From the HTML 4 specification:
ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").
The good news is that the HTML 5 specification is more lenient:
The
idattribute specifies its element's unique identifier (ID). The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character. The value must not contain any space characters.
Why can't I have a numeric value as the ID of an element?
Because that's what the HTML4 spec dictates.
On the other hand, the HTML5 spec has removed this requirement.