The Protestant answer (excluding high-church European Reformation 1.0 churches like Anglican and Lutheran) will be simply this: Because an altar is for sacrifice, and Jesus made the once for all sacrifice. So rather than offering a sacrifice ourselves on an "altar" we simply remember Jesus' sacrifice around a "table," the Lord's Table (thus Paul calls it in 1 Cor 10:21).
1 Cor 10:21 Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord's table, and of the table of devils.
Even the Catholic church, post Vatican II, is coming around to this view. You can see that instead of the old "high altars" they often tend to have much simpler tables. Instead of the priest standing ad orientem (toward the East, the back wall of the church where the high altar was, and hence away from the congregation) the priest stands in front of a simple wood table and faces the congregation while administering the Eucharist. (A traditional Catholic centerpiece for the old high altars, the "tabernacle" which contains the reserved host, which Catholics believe is the body of Christ, at times when the Mass is not being celebrated, and to which the priest would bow at several points, was often not clearly visible in this setup, although Roman Catholic Canon law prescribed
The tabernacle in which the blessed Eucharist is reserved should be sited in a
distinguished place in a church or oratory, a place which is conspicuous,
suitably adorned and conducive to prayer.
[Canon 938 §2 of the Code of Canon Law, 1983])
It's one of the things that really riles the "traditionalist" Catholics up against the Novus Ordo mass. They perceive the Novus Ordo as favoring table-language and table-orientation, while the Tridentine mass (Traditional Latin Mass [TLM], also now called Extrordinary Form [EF]) favored the ad orientem (towards a high altar) orientation.