That has been studied. In fact, it used to exist in the act, but not for international voters. Before 1993, there was proxy voting, but it was decided that it would be best to get rid of it.
I'm going to posit two reasons for that decision: first, the notion of the secrecy of the vote; and second, the leap of faith that voters make when they entrust their vote to someone else. Those are probably the two main flaws of proxy voting.
That said, there's also an administrative burden that comes with confirming that the proxy does indeed have the mandate from the voter, which can take time. The debate could be reopened, but at the time, it was decided that as long as there were administrative steps, it was preferable to expand the special ballot rather than pursue proxy voting and expand it internationally.