Josephus writes:

He also made some of them to be eunuchs... Now among these there were four of the family of Zedekiah... one of whom was called Daniel.

By stating that the king made "some" of the captives eunuchs and then immediately listing Daniel and his three friends as the notable members "among these," Josephus implies they were eunuchs.

The Bible does not record these men as ever having wives or descendants, so that would explain why.

Also, the prophet Isaiah said this would happen, that people of Israel would be made eunuchs and serve a foreign king.

Isaiah 39:5-7 KJV | Then said Isaiah to Hezekiah, Hear the word of the LORD of hosts: Behold, the days come, that all that is in thine house, and that which thy fathers have laid up in store until this day, shall be carried to Babylon: nothing shall be left, saith the LORD. And of thy sons that shall issue from thee, which thou shalt beget, shall they take away; and they shall be eunuchs in the palace of the king of Babylon.

Daniel and the three Hebrew boys served in that palace! So It looks highly likely they were eunuchs.

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Antiquities of the Jews - Book X, Chapter 10
https://www.earlyjewishwritings.com/text/josephus/ant10.html