Because weekly confession is not necessary for salvation, nor is it physically possible for all Catholics. There are 400,000 priests in the world. If every confession was just 3 minutes long on average, absolving every Catholic weekly would require all priests to hear confessions 150 hours per week.
Confession is only necessary for salvation if one is committing mortal sin. If you are committing mortals sins every week, you probably have some pretty serious life issues, and just going to confess them all the time is probably not the answer. Confession is only effective if you've already resolved to change to life and are actually doing so.
Its nice to go to confession just for venial sins weekly or monthly, but we need to be aware of laying up burdens upon people that are apart from the Gospel. Regular reception of the Eucharist, and regular prayer, are more than sufficient to cure most of our venial faults.
We need to keep in mind that the first Christians managed to make it through life with usually just a single opportunity for confession after Baptism. Frequent confession was not widely practiced until at least 1000 years after the time of Christ.