Because discipleship is essential for salvation and baptism officially initiates discipleship, much as a marriage ceremony does for marriage. There are marginal cases where a marriage might exist without an initiating ceremony but that doesn’t make us preach against weddings. Likewise, if ignorance or some other crisis prevents a disciple from being baptized this does not make baptism normally extraneous.

Because faith is essential to salvation and faith must believe a message as offered. So when the message is, “Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins…” submitting to baptism is not an addition to faith, but an expression of faith. Refusing baptism in such circumstances is unbelief, and unbelievers remain under God’s wrath.

Thus Francis Turretin:

Although the sacraments are external means and instruments applying (on the part of God) the promise of grace and justification, this does not hinder faith from being called the internal instrument and means on the part of man for receiving this benefit offered in the word and sealed by the sacraments [16.7.20].

The question is not whether faith alone justifies to the exclusion either of the grace of God or the righteousness of Christ or the word and sacraments (by which the blessing of justification is presented and sealed to us on the part of God), which we maintain are necessarily required here; but only to the exclusion of every other virtue and habit on our part…. For all these as they are mutually subordinated in a different class of cause, consist with each other in the highest degree [16.8.5].