The commandment to be fruitful and multiply -- פְּר֥וּ וּרְב֛וּ [Gen. 1:28] appears to have been issued to all men [Yevamot 6:6], not just Jews. So why isn't it one of the Noahide laws, which apply to all humanity?
The simple answer is that not every man is included in the injunction.
Whilst the Sefer HaChinuch writes:
ונוהגת בכל מקום ובכל זמן (קדושין לו, ב) וחיב אדם להשתדל בה משהוא ראוי לה, והוא הזמן שנתנו חכמים ז''ל (אבות פ''ה מכ''א) לשא אשה. ומצוה זו אינה מוטלת על הנשים, והמבטלה בטל עשה וענשו גדול מאד, (קדושין כט, ב) שמראה בעצמו, שאינו רוצה להשלים חפץ השם לישב עולמו.
And [it] is practiced in every place and at all times (Kiddushin 36b), and a man is obligated to be involved with it from when it is fitting for him, and this is the age which the sages (Avot 5:21) designated to marry a woman. And this commandment is not incumbent upon women. And one who negates it violates a positive commandment; and his punishment is very great (Kiddushin 29b), as he shows that he does not want to fulfill God's will to settle His world.
The Minchas Chinuch writes pretty definitively:
ומצוה זו. מבואר ביבמות ס"ה ור"מ פט"ו מה"א דב"נ אינו מצווה בפ"ו ועבד כנעני אי מצווה אינו מבואר בר"מ ועיין תוס' חגיגה ד"ב ע"ב ד"ה לא תהו כתבו בשם י"מ דעבד אינו מצווה על פ"ו...
And this commandment - It is explained in Yevamos 65 and the Rambam (Hilchos Ishus chapt 15), Bnei Noach are not commanded in the commandment of 'Be fruitful and multiply', and with regard to a Canaanite slave and whether they are commanded, it is not explained in the Rambam, but Tosafos in Chagigah 2b (s.v. 'He did not create it a waste') it writes there that there are those who say that they (i.e. a non-Jewish slave) are not commanded to be fruitful and multiply...
Tosafos Sanhedrin 59b explains the reason it's not listed in the 7 mitzvos is because active mitzvos aren't included. Only negative commands. However Tosafos says it's also a negative command not to waste seed. Tosafos implies that it's not included in the 7 mitzvos because they don't have it nowadays. Gemara 59b says that non jews lost the mitzva during matan Torah, so now only jews have it.