The selection of the writings of Holy Scripture, in which you search for direct reference to the Immaculate Conception, was done by Holy Spirit-inspired Catholic men.
The writings by Church Fathers about Mary, which did form Catholic tradition and the Magisterium and the papal decree, were done by Holy Spirit-inspired Catholic men.
The “reason” you write of, within us Catholics, is faith.
If, as you claim, Catholics wrote the Bible, why don’t you use it for the immaculate conception? Because you can’t. So you proceed to divert from the issue. Catholic encyclopedia admits you can’t justify this false dogmas with scripture or “tradition”.