And let's not forget that the whole idea of catechisms and catechumens came from the Catholic Church, which also put tradition on a par with Scripture.
Furthermore doctrines should be DERIVED from Scripture, not imposed upon Scripture within a predetermined framework. Reformed Theology (or Catholic theology) is within a predetermined framework.
Actually, the first formal catechism was written by Martin Luther in 1526, to instruct lay people in the basics of Biblical teaching. (Remember that literacy levels were low at the time, there was as yet no German translation of the Bible, and ignorance of doctrine was almost universal). The question-and-answer format enabled even poorly-educated people to learn doctrine and memorise it easily. The first Catholic catechism didn't appear until almost 40 years later.
A catechism is a tool - nothing more, nothing less. Of course all spiritual truth is to be found in the Bible - but it's scattered around and takes a lifetime to fully absorb. A good catechism
is derived from Scripture. It isn't a substitute for Bible study; but it helps new believers to get their heads round the basics quickly.
The reason that catechisms vary in the doctrine they contain is that Christians and churches also vary in their beliefs! All are based on Scripture, the differences are in our interpretations. So if you are opposed to certain Reformed teachings you won't want to use a Reformed catechism (just as Protestants don't use the Catholic catechism). It's a bad idea to approach any catechism with the belief that it imposes a framework onto Scripture - that way you do end up studying the catechism itself and not the Bible. But as a tool, it can be useful when used for its proper purpose.