Schema-awareness and built-in types

The following doesn't work in an XSLT processor with basic conformance:

<xsl:element name="a" type="xs:integer">5</xsl:element>

even though you can create variables using the built-in types:

<xsl:variable name="a" select="5" as="xs:integer"/>

or use the constructor functions:

<xsl:variable name="a" select="xs:integer('5')"/>

What's the reason for this restriction? Why can't I use built-in types
to annotate new nodes?
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Max Toro