Second Thessalonians 2:15
“Therefore, brothers, stand firm and hold to the teachings we passed on to you, EITHER by word of mouth OR by letter"
One of the great differentiating factors between Catholicism and many Protestant faiths is the equal importance of Tradition and The Bible.

In the case of infant Baptism, it was a thing that was just always done, and always just deemed common sense that no one ever would have seen the need to write it down. Akin to how nowhere in The Bible does it explicitly say Marriage is between a man and a woman; it was just beyond reason that someone would think any other way, so no one ever would have seen the need to write it down.

Recall that there was no written compilation of the New Testament for hundreds of years after the death of Christ. All of Christian teaching was passed down orally and through tradition; there was no written word. This is why the older Christian faiths believe things that are not explicitly stated in either the Old or New Testaments. These were things that the Apostles and disciples of Christ saw Jesus say or do but were, at the time, so common- sense to them that it never occurred to any of them to write down explicitly.

These traditions were passed on orally until there became a need to formally document the Traditions as the Church expanded into cultures and times for whom centuries-old Aramaic Hebrew traditions were no longer common sense.

Infant baptism was one of these traditions that simply "always was" from the time of Jesus and his apostles. Newer Protestant denominations abide by varyingly strict interpretations of Sola Scriptura="By the Scripture Only", that throws out the concept of Tradition, and that only what is explicitly stated in The Bible should be followed. And since The Bible did not explicitly state "Infants can/ should be baptised", they do not consider it legitimate.

Obviously this topic of Tradition vs Sola Scriptura goes much deeper, but this should explain why some Protestants have negative (and sometimes even hostile!) reactions to infant Baptism as practiced by the earlier Christian Churches who value Tradition as equal to The Word.

God Bless you!