Because CTEs are not named subqueries, really. For example, CTE is only evaluated once, even if it's referenced in multiple places, and those places may have conflicting ideas of what's the best way to evaluate it.

Consider for example this:

WITH x AS (SELECT a, b FROM t) SELECT * FROM x x1 JOIN x x2 ON (x1.a = x2.b);

Now, had this been evaluated using a merge join, the "x" CTE would have to be sorted first by "a" or "b" (for either side of the join). Well, that can't really happen.

Another issue is locking - consider this version, for example:

WITH x AS (SELECT a, b FROM t FOR UPDATE) SELECT * FROM x x1 JOIN x x2 ON (x1.a = x2.b);

In other words, it's way more complicated than it might seem, especially if you can't break existing uses (e.g. the locking).