“Summa Theologica” was an attempt at an entire cyclopedia of Western Christian (i.e., Catholic) theology. The Catholic Church *is* the scripture-based denomination; documents like the catechism refer to Church Fathers simply because the Church Fathers explain the scripture.
Incidentally, this scriptural basis didn’t stop Thomas from theological speculation. IIRC, Summa Theological is the work which is the source of the legend of arguing how many angels can fit on the head of the pin. (The actual argument is whether two or more angels can occupy the same physical space, which, in turn reflects whether angels have volume, which in turn reflects whether angels exist in what Einstein would call time-space... so while it’s speculative, it’s hardly as inane as Disraeli’s satire makes it to be.)
Actually, the Summa is theology for beginners.
"Because the doctor of Catholic truth ought not only to teach the proficient, but also to instruct beginners (according to the Apostle: 'As unto little ones in Christ, I gave you milk to drink, not meat' -- 1 Corinthians 3:1-2), we purpose in this book to treat of whatever belongs to the Christian religion, in such a way as may tend to the instruction of beginners."
-- Summa Theologiae, Prologue