It wasn't deemed required; for example the Director of the IMF’s European
In sum, the euro’s architecture was built on the premise that market forces,
combined with minimal coordination of policies, would suffice to align
economies, discipline fiscal policies, and allow countries to withstand
The gist of that is echoed here:
what I recall from discussions at the time was the belief that two factors
would make the adjustment problems manageable. First, countries would adopt
sound fiscal policies, and thereby reduce the incidence of asymmetric
shocks. Second, countries would engage in structural reforms that would make
labor markets— and, presumably, wages—flexible enough to cope with such
asymmetric shocks as occurred despite the soundness of the fiscal policies.
And this paper mentions:
The proponents of the second perspective, which has been dubbed the
‘endogenous currency area’ (ECA) argument (alternatively, the ‘self-
validating currency area’ argument) [..] assert that the creation of a
currency area can itself induce changes that actually enable the participating
countries to achieve a sufficient degree of integration [..] ex post to make a
currency area viable [...]. ECA proponents argue that greater optimality
arises because adopting a single currency typically has expanded trade
dramatically with other currency union members .
and the ECB reported in
If national fiscal stabilisers work in the wake of adverse
shocks, the need for other types of adjustments -- such as supranational
transfers, international risk sharing through the financial system, changes in
prices and wages, and/or changes in real exchange rates -- are somewhat reduced.
Note that these viewpoints were somewhat controversial even at the time (the
second and third links are criticisms of this viewpoint).
It also fits with how much of the EU's was constructed. As Robert Schuman put it
in the Schuman Declaration:
"Europe will not be made all at once, or according to a single plan. It will be
built through concrete achievements which first create a de facto solidarity."
The Euro was effectively a continuation of the European Currency Unit launched in
1979. While I can't find a conclusive reference for it, I would imagine that the
creators of the Euro imagined further integration in some form at an unspecified
future point. Thus far such integration has largely failed to materialize due to
various reasons outside of the scope of this answer.