The premise of your question may be inaccurate. The Mishnah was also a ‘book’, however, it is basically just quoted as teachings. So too the Yerushalmi, there are countless examples of parallel teachings and stories etc. in the Bavli from the Yerushalmi.
Additionally, as @kouty said the phrase אמרי במערבא may denote how they referenced the Jerusalem Talmud. Perhaps it is not meant to be taken literally.
Here are some references:
Talmud Bavli, Chagigah 10a, with Rashi, d.h miTalmud l’Talmud:
אמר רב כיון שיוצא אדם מדבר הלכה לדבר מקרא שוב אין לו שלום. ושמואל אמר זה הפורש מתלמוד למשנה. ורבי יוחנן אמר אפילו מתלמוד לתלמוד.
רש״י - אפילו מתלמוד לתלמוד – מתלמוד ירושלמי לתלמוד בבלי שהוא עמוק, כדאמרינן בסנהדרין (כד, א) במחשכים הושיבני כמתי עולם זו התלמוד של בבל.
Rav said: “As soon as man goes forth from the study of Halacha to the study of Scripture he no longer has peace. And Shmuel said: “It means one who leaves Talmud for Mishnah.” And Rav Yochanan said: “Even from Talmud to Talmud.”
Rashi explains: “from Talmud to Talmud: from Talmud Yerushalmi to Talmud Bavli, since the former one is deeper.”
And a more indirect statement showing their awareness of the Jerusalem Talmud style of teaching on Bava Kama 6b:
כשהזיק חב המזיק חב המזיק חייב המזיק מיבעי ליה אמר רב יהודה אמר רב האי תנא ירושלמי הוא דתני לישנא קלילא
§ The mishna states: And when a component of any of these categories causes damage, the owner or generator of the component that caused damage is obligated [ḥav] to pay restitution with best-quality land. The Gemara asks: Why does the tanna of the mishna use the unusual expression: The one liable for the damage is ḥav to pay? He should have used the more standard expression for liability: The one liable for the damage is ḥayyav to pay. The Gemara explains: Rav Yehuda said that Rav said: This tanna is a Jerusalemite, who teaches using a light and concise formulation, as ḥav is more concise than ḥayyav.