Because those of us who rightly recognize baptism as a sacrament also recognize that it is a work of God and know that His work is always perfect. We can trust Him to get it right.
Those who regard it as their own good work are left with the nagging question of whether it was good enough. So they invent baptism laws not found in the Bible as "hoops to jump through" to convince themselves that their baptism was indeed valid.
It is very ironic that those who claim a simple faith, often make it complicated. Similarly, Catholics are accused of "works salvation" but do not have requirements to read the KJV Bible daily, dress only in dresses, only homeschoool, etc. If you can say "If you were really a Christian, then you wouldn't/would . . . " then you are looking to works.
The form of baptism does not seem to have been strict even in the earliest days of Christianity. Here is a passage concerning baptism from the Didache, which was written about the same time as the Gospel of St. John:
Chapter 7. Concerning Baptism. And concerning baptism, baptize this way: Having first said all these things, baptize into the name of the Father, and of the Son, and of the Holy Spirit, in living water. But if you have no living water, baptize into other water; and if you cannot do so in cold water, do so in warm. But if you have neither, pour out water three times upon the head into the name of Father and Son and Holy Spirit. But before the baptism let the baptizer fast, and the baptized, and whoever else can; but you shall order the baptized to fast one or two days before.
What is important, is that there is water, and the Trinitarian formula. Whether the water is "living" from a natural source such as a river, or from a font in your church, whether it is cold or warm, whether it is immersion or poured, these things are not so important.