To quote the GNU make info pages:
-------
A "substitution reference" substitutes the value of a variable with
alterations that you specify. It has the form `$(VAR:A=B)' (or
`${VAR:A=B}') and its meaning is to take the value of the variable VAR,
replace every A at the end of a word with B in that value, and
substitute the resulting string.
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I.e., "VAR" has to be the name of a variable. So, given
all: $($(wildcard *.page):.page=.html)
make expands "$(wildcard *.page)" *and then treats the result as the
name of a variable*. Since your makefile doesn't have a variable with
the name "whatever.page another.page something.page etc.page", the
result is empty.
To do a pattern substitution on something other than the result of a
variable expansion, use the $(patsubst) function directly:
all: $(patsubst %.page,%.html,$(wildcard *.page))