Q. Wouldn’t it make sense to celebrate the completion of our reading of the Torah on Shavu’ot, when the Torah was given on Sinai?
The prosaic answer is that the two festivals celebrate different things – Shavu’ot marks the giving of the Torah, whilst Simchat Torah celebrates the end (and beginning) of the annual cycle of readings.
A well-known theory adds that even if we had thought of combining the two occasions, it would not have worked.
It is like a couple who get married; on their wedding day their instinct tells them that they are right for each other, but it takes time to find that this is really the case.
Likewise, when on Shavu’ot Israel and the Torah became, as it were, wedded to each other, neither was completely sure of the other. It took time for them to get to know one another and to feel able to rejoice without any lingering doubts. Hence, by Simchat Torah, several months on from Shavu’ot, we know we and the Torah were made for each other and we can rejoice with heart, soul and might.