I have a bit of a Lutheran answer to the question of why ... and a practical observation.
First, from my brief study of Lutheranism (because the Church du jour that we attended when I was born happened to be Lutheran, so I was infant baptized Lutheran and had some innate sense of loyalty):
Like Catholics, they take the elements very seriously as well (the original transubstantiation vs trans-substitution debate).
Historically, if a man spilled the wine on his beard, the correct response was to shave off the beard and burn it as an offering to God ... it was sanctified by contact with the sacred 'wine'.
1 Corinthians 11:
27 So then, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. 28 Everyone ought to examine themselves before they eat of the bread and drink from the cup. 29 For those who eat and drink without discerning the body of Christ eat and drink judgment on themselves.30 That is why many among you are weak and sick, and a number of you have fallen asleep. 31 But if we were more discerning with regard to ourselves, we would not come under such judgment. 32 Nevertheless, when we are judged in this way by the Lord, we are being disciplined so that we will not be finally condemned with the world.
A good pastor (shepherd) of a local body of the people of God, might want to protect his 'flock' from the danger of drinking "the cup of the Lord in an unworthy manner".
Thus the sacrament of the Lord's Table is carried out as an assembled body (even my Southern Baptist friends would approve) and each person participates personally by eating the wafer and the entire body participates corporately in the drinking of the wine ... without the risk of "an unworthy manner".
[Note that these are not my personal beliefs, just an attempt to honestly present the beliefs of another without assuming a bad motive that might not be their intent.]
A more practical look at problems with using real wine for everybody.
First, there is a modern sanitation issue ... would you want to be the 53 of 125 people who was about to sip from the same cup?
Jesus broke one piece of Matzo (unleavened bread) and handed out the pieces ... the modern sharing of the 'body' is far less 'hand on' and far more 'sanitary'.
I would expect that the same modern sensibilities would apply to the drinking of the 'wine'.
[As an aside, our modern ceremony as commonly practiced in churches (including mine) has lost some of the power of the original symbolism ... 12 men ate from the same piece of 'bread', one spiritual food (Jesus) fed, indwelt and united all making them almost literally one body with Christ as its head/source ... like wise the cup of the new covenant was one literal cup that all drank from ... we have traded some of the original visual reinforcement of spiritual truths for modern convenience and hygiene.]
So why not use real wine in those little plastic cups?
In most churches it is a very practical reality that people as young as 13 can have made a public confession of faith, become members of the church, and be expected to participate in communion.
In most states the legal drinking age is 18 (21 in some states) ... making it illegal for underage church members to take communion with the rest of the body of believers.
The legal issue has two obvious solutions ...
1) only the priest/shepherd/pastor drinks real Wine (in which case, the people do not drink as commanded by Jesus).
2) everyone drinks grape juice (not wine as Jesus commanded).
I can see men of faith following their conscience to either solution ... then
1 Corinthians 10:31 "So whether you eat or drink or whatever you do, do it all for the glory of God."
As a personal observation, the Catholic Church that I attended in the early 1980's did not serve wine to the people ... not a statement about the whole denomination, just about that 1 specific building and the people who attended services there.