The simple answer is that they aren't obligated (Sanhedrin 59b).
As a general rule for understanding how the Talmud views learning these things from verses, anything said exclusively before Sinai is only for Jews (because there can't be anything that Jews are not obligated in but non-Jews are), but if it is said before Sinai and after Sinai, then it applies to non-Jews (as well as Jews) (this is covered in Sanhedrin 59a-b). And then there are things that are said in both places that only apply to Jews for technical reasons (of which Puru U'Rvu is one).
All that being said, there are more obligations to non-Jews than just the seven. The seven kind of correspond to the 613. There are more obligations than the 613, but there is a meaning and reason behind that specific classification. In this case, one of the rules for inclusion in the 7 is that it has to be a negative commandment ("sit and don't do") - Sanhedrin 58b at the very bottom. Establishing courts of justice is included because it has a negative aspect (the Talmud - Sanhedrin 59a at the very top - doesn't elaborate what that is, but Rashi explains it as VaYikra 19:15 - לא תעשו עול במשפט - do not do injustice).
The Talmud (Chulin 92a at the bottom) refers to 30 Mitzvos for Bnei Noach, and some have claimed you can get up to 66. The whole question isn't well explored for historical reasons (there were limited practical Halachic questions in this regard over the years).
So the upshot is: Puru U'Rvu is not a Mitzvah for Bnei Noach (at least according to that part of the Talmud, I don't know if that is the conclusion - so CLOR), even if it were it wouldn't be counted because it is a positive obligation (however, see the first Tosfos Sanhedrin 59b where it includes a negative aspect, but that follows the opinion that it is not a Mitzvah for Bnei Noach), and as long as it is a positive obligation there are many more potential obligations beyond the 7 (plus the Talmud in Sanhedrin lists individual opinions that include more than the 7 in the negatives).
The answer is because it was not repeated at Sinai, and any mitzvah which was said only once but not repeated is understood to be exclusively a Jewish mitzvah. (מצוה שנאמרה ולא נשנית, לישראל נאמרה ולא לבני נח--סנהדרין נט ע"א). The reason for this is that there is nothing which is mutar for a Jew which is prohibited for a non-Jew (Sanhedrin ibid.).
The Minchas Chinuch, mitzvah #1 mention one source who claims that mankind were indeed commanded (and obligated) in Peru Urvu but only until Matan Torah when this mitzvah was taken away from them (and put on the responsability of Am Yisroel). The reason is, perhaps, that once mankind's population increased so much they didn't need it anymore and since then the mitzvah only apply to jewish people who were small in number (Devorim 4:27 and 7:7).