Many beliefs of the Church existed from the time of Jesus, and preceded the Scriptures, in many cases by 150 or 200 years. You say that the Eucharist is not in Scripture. We say it is and have the verses that we use to justify it.
The fact is the Catholic Church has included works of other authors that was not considered infallible-relying upon documents subject to error, to justify erroneous positions. It would be the same as if Augustine quoted Jerome giving it the same status as the infallible scriptures. This would never have occured to them.
Individual works of Church Fathers which do not meet the Magisterial approval are not considered infallible. Works that are approved are brought into the Church and considered part of the Church. Jerome was correct in many ways, as well. That does not mean that all Jerome was correct.
If you're equating the Eucharist with communion, then no Protestants I know of would say communion is not in scripture. However, what is in question is the purpose of communion. Catholic teaching is that the Eucharist is a means of imparting God's grace. There is nothing in scripture that supports this notion. What is in scripture is the Lord Supper proclaims our Lord Jesus' death and is a memorial until He returns.
1Co 11:25 In the same way also he took the cup, after supper, saying, "This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me."
1Co 11:26 For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes.