The actual answer to this is "History". The women's heptathlon was only introduced in the Olympic program in 1984; before that women contested a pentathlon (five events) when they were allowed to compete in multi-events at all.
Bear in mind that almost all advances toward event equality between men and women have happened in the last half-century, and that the glacial pace of change in international bureaucracies still dominated by older men means there is still work to be done. 1984 was also the year of the first women's marathon, and long-distance track races (e.g. 5,000m/10,000m) didn't achieve parity until 1996; the women's 3000m steeplechase didn't make its Olympic debut until 2008.
Taken as an aggregate, there are physiological differences between men and women (e.g. leg length - the average man has longer legs than the average woman) which lead to different event specifications, such as the height of hurdles and the distance between them.
In an event with as many variables as the multis have (event distances, hurdle heights, implement weights, etc.) the negotiation of which of these variables are affected by measurable physiological differences and which are simply holdovers of history gets really, really complicated.