Praise beIn the name of God, the Beneficent, the Merciful
All perfect praise be to God, the Lord of the worlds. I testify that there is none worthy of worship except God and that Muhammad  is His slave and Messenger.
Your question: Why are Muslims not allowed to make friends with Jews and Christians but you are allowed to marry them?
Answer:
Taking them as friends in general terms, taking them as helpers and supporters, and throwing in one’s lot with them. God forbids all this, as He says (interpretation of the meaning): ”O you who believe! Take not the Jews and the Christians as awliya’ (friends, protectors, helpers, etc.), they are but awliya’ to one another…” [al-Maa’idah 5:51]. Compromising with them and being nice to them at the expense of one’s religion. God says (interpretation of the meaning): ”They wish that you should compromise (in religion out of courtesy) with them, so that they (too) would compromise with you.” [al-Qalam 68:9]. (islamqa.info)
Actually,As you misunderstoodcan see in here, it’s not allowed the reason for this, is because, you might fall into disbelief as per the Quran. Also, the Prophet asked the polytheists( mushrik) if they believe in God and in His prophet and he or they replayed negatively and he suggested by telling them, “Go back, I do not need your help”. Hence, it is much better to take as friend, advisor, protector etc... a believer instead of polytheists and atheists. But, you can be friend even with disbelievers as long as they don’t go against your religion as per the verse:
”O”God does not forbid you from those who believe! Takedo not the Jewsfight you because of religion and the Christians as Awliyaa’do not expel you from your homes (friends, protectors- from being righteous toward them and acting justly toward them. Indeed, helpers)God loves those who act justly. (60:8)
God Says:
”This day [all] good foods have been made lawful, they are but Awliyaa’and the food of each otherthose who were given the Scripture is lawful for you and your food is lawful for them. And if any amongst[lawful in marriage are] chaste women from among the believers and chaste women from among those who were given the Scripture before you takes them (as Awliyaa’), then surely, he is one ofwhen you have given them. Verily their due compensation, God guidesdesiring chastity, not those people who areunlawful sexual intercourse or taking [secret] lovers. And whoever denies the Zaalimoonfaith (polytheists and wrongdoers- his work has become worthless, and unjust)he, in the Hereafter, will be among the losers.” (5:5)
In here it meanswe learn that, if you can takewant to marry a Christian or JewJewish lady or any kind of disbelievers as your best friend with the following conditions:
Your Christian or Jew friend should not be taken against Muslims
You only can take Christian or Jews as your friends who do not mock or make fun or insult any element of Islam.
You can take any kind of disbelievers as your friend as long as he or she is treating you with respect, and does not do anything that goes against Islam.
You can take any kind of disbeliever you want as long as this person does truly respect the ways, believe etc.. of Islam.
You can take any kind of disbelievers as long as they are truly not against you.
Also prophet Muhammad married a Christian woman name Marie and then she became a Muslim. So according to this, you can be friend with anyone you wishlady, either with an atheist or anybody you wish. And of course, when we read a Hadith, we need to try to understand it instead of misunderstand it.
And God knows the bestare allowed.