The Gemara in Megillah 7b is clear:
חייב איניש לבסומי בפוריא
A person must smell good in bed
However, I have seen that people are not careful about this. In fact, I have known some Yeshiva students who are incredibly scrupulous in other areas of Halacha go for days without showering! Why has this halacha been left by the wayside?
They follow the opinion of Hillel (Vayikra Rabba, 34:3), that bathing is merely a mitzva and not an obligation:
הלל הזקן בשעה שהיה נפטר מתלמידיו היה מהלך והולך עמם אמרו לו תלמידיו ר' להיכן אתה הולך אמר להם לעשות מצוה אמרו לו וכי מה מצוה זו אמר להן לרחוץ בבית המרחץ
The Rambam (Hil. Talmud Torah, 3:4) rules that Torah study takes precedence over other mitzvos when those other mitzvos can be performed by other people:
היה לפניו עשיית מצוה ותלמוד תורה אם אפשר למצוה להעשות ע"י אחרים לא יפסיק תלמודו. ואם לאו יעשה המצוה ויחזור לתלמודו
Since there are other people who can shower before bed, yeshiva students need not interrupt their studies to shower.
Further, note the choice of verb conjugation in Rava's statement: לבסומי is not reflexive, so Rava's statement can be fulfilled by spraying air freshener or burning incense while in bed.
You're quoting out of context. Here it is in full:
אמר רבא מיחייב איניש לבסומי בפוריא עד דלא ידע בין ארור המן לברוך מרדכי
Rava said: A person is obligated to smell good in bed, unless he knows [the difference] between "cursed is Haman" and "blessed is Mord'chay".
Presumably, whoever you meet who doesn't shower does know the difference.
Or doesn't sleep in a bed.
You are misreading the Aramaic. The word is "איניש " which is Aramaic for "punish."
There is an obligation to punish if he goes to bed smelling good.
The Gemara states that after Rava said this halacha, Rabbah got drunk and killed R' Zeira. The Ran quotes the R' Efrayim that this teaches us that the Gemara rejects Rava's ruling.
