The idea of a constitutional monarchy was among the many floated at the time but it wasn't realistic for a couple of reasons:

• The japanese had deliberately mixed japanese blood into the royal line by forcing the crown prince to marry a minor japanese aristocrat and similar with their descendants. That was an attempt to preempt rebellions centered around restoring the crown. It made sense from the japanese mindset that the population submits once the leadership is defeated but it didn't work in korea because the rebellions were about pushing out the japanese not restoring the royal line.

Calling to restore a royal line that was now more japanese than korean wasn't going to be either popular or palatable after you'd spent two generations fighting to liberate yourself from the japanese.

• The population was by that point too politically aware to be interested in a monarchy. As soon as news of the japanese surrender spread the people organised village councils for self governance and the whole spectrum of political parties (hundreds IIRC) spouted.

All that ended when the japanese collaborators were put back into power by the US military against the people's wishes. All the massacres and "anti-communist" repression that you might hear about in early ROK history were about putting down political rivals, i.e. left, centre, and rightist parties not associated with the US's chosen leader (Rhee SeungMan) ceased to exist.

Indirectly, Rhee SeungMan's elevation to leadership by the US killed any alternatives to anything other than a US style presidential system.

The idea of recreating a symbolic monarchy comes up every so often but it's more about pomp & ceremony than a real part of the political system.