Why do Muslims, Jews and Christians have beards when it is a holy Sunnah for Muslims?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Originally the Muslim grows his beard for the Hadith: "Trim your mustaches and leave your beards (grow)". [Reported by Imams Al Bukhari and Muslim]. As for your saying that the Jews and Christians grow beards, I think this generalization is not accurate. In fact, only a few of them grow beards. In addition, growing the beard is one of the ten Fitra (acts natural predispositions). The Prophet said: "Ten are among the Fitra: trimming the mustache, growing the beard, brushing one's teeth, sniffing water, cutting one's nails, washing one's knuckles, depilating the armpit, shaving the pubes and using water to remove impurities (urine and excrement) from the body".
The scholars explain Fitra as the creation, and what is meant by Fitra here is the ancient Sunnah which Allah chose for His messengers. So, it is a set of natural acts which were called for by all messengers.
So, since growing beards is a Sunnah in all the monotheist is religions, some Jews or Christians might grow beards as an imitation of their messengers or a return to the Fitra.
Another reason men, in general, grow a beard is that it is an embellishment; for men as mentioned by the Mother of Believers, Aisha (), who said: "Glory to Allah, He embellished men with beards and women with plaits". [Narrated by Al Hakim].
Anyway, whether Jews and Christians grow beards or shave, this will not bring them any benefit as long as they do not become Muslims and believed in Prophet Muhammad (Peace and blessing be upon him). Allah Says (interpretation of meaning): {And whoever seeks a religion other than Islâm, it will never be accepted of him, and in the Hereafter he will be one of the losers.}[3:85].
Allah knows best.