Ok hear me out first. If Betulah really means virgin, my question is why does Genesis 24:16 which uses this word to describe Rebekah, feels the need to also say "no man had known her" וְאִ֖ישׁ לֹ֣א יְדָעָ֑הּ?
Betulah specifically means a woman whose hymen is intact. The hebrew word for a woman's hymen is בְּתוּלִים (betulim), from the same root. This is borne out of a careful read of the following verses.
First, note that the passage in Deuteronomy 22:13-21 about a man who defames his wife, hinges on the woman having had a hymen. The man comes and says "אֶת־הָאִשָּׁ֤ה הַזֹּאת֙ לָקַ֔חְתִּי וָאֶקְרַ֣ב אֵלֶ֔יהָ וְלֹא־מָצָ֥אתִי לָ֖הּ בְּתוּלִֽים." This means that he slept with her after their wedding and found that she did not have betulim, an intact hymen, and therefore must have cheated on him. Her parents then respond by וְהוֹצִיאוּ אֶת־בְּתוּלֵי הַֽנַּעֲרָה (lit. bring out the betulim of the young woman) bringing out evidence in the form of a bloody sheet that she was a virgin on their wedding night. This determines the meaning of בתולים betulim as the hymen.
Next, note the Kohen Gadol is commanded to marry only a specific kind of woman, described twice in the passage: first in Leviticus 21:13 as אִשָּׁה בִבְתוּלֶיהָ יִקָּח which literally means "and a women with her betulim he should take", and then in the next verse the requirement is repeated as "כִּ֛י אִם־בְּתוּלָ֥ה מֵעַמָּ֖יו יִקַּ֥ח אִשָּֽׁה a betulah from his people as a wife". The parallel structure indicates that בתולה betulah is equivalent to אשה בבתוליה a woman with her hymen.
There remains your question about Genesis 24:16 (or other places where the word "betulah" isn't used to indicate virginity, such as Leviticus 21:3 or Numbers 31:17-18). Rashi on your verse explains that betulah only means having an intact hymen, whereas וְאִ֖ישׁ לֹ֣א יְדָעָ֑הּ "and no man had known her" comes to exclude other types of sexual interaction that don't involve breaking the hymen. Rebekah had not engaged in any such activity.
Тhere is a difference here between the language of the Torah (the Pentateuch) and the language of the Sages.
In case you think this is the plain meaning, see Rambam, Mishneh Torah, The Laws of Forbidden Intercourse, Ch.17:מצות עשה על כוהן גדול, שיישא נערה בתולה; ומשתבגור, תיאסר עליו: שנאמר "והוא, אישה בבתוליה ייקח" (ויקרא כא,יג)--"אישה", לא קטנה; "בבתוליה", לא בוגרת.
The primary meaning of בתולה/betulah is "a young maiden." It does not necessarily carry the meaning of virgin without additional qualifying language to add that aspect to it.
The Akkadian cognate batultu is discussed in J. J. Finkelstein, "Sex Offences in Sumerian Laws," JOAS 86 355:72, affirming that the related word in that Semitic language "denotes primarily an age group." This is also true of the other Semitic languages, e.g. Sumerian, Ugaritic, Aramaic....
In the Semitic languages, there is no lone word that connotes a virgin; rather, that quality is conveyed by the phrase "who has not known a man" or, as in Bereshit (Genesis) 24:16, "no man has known her." Such qualifiers also occur in Vayyiqra (Leviticus) 21:2-3 and Shofetim (Judges) 21:12. These qualifiers, as your question implies, would be superfluous if the word on its own means "virgin."
There are certainly Scriptural instances where a meaning of virgin cannot be in view, e.g. Yoel (Joel) 1:8, where the betulah is lamenting over the death of her husband or in Esther 2:17, where the betulot referenced are King Achashverosh's concubines.