Actually, you will probably see a lot of prelates vested “in choir” on Monday for the Papal Mass. These are generally cardinals and archbishops who will be present for the Mass, but, not concelebrating.
Here is what Redemptionis Sacramentum notes:
[128.] Holy Mass and other liturgical celebrations, which are acts of Christ and of the people of God hierarchically constituted, are ordered in such a way that the sacred ministers and the lay faithful manifestly take part in them each according to his own condition. It is preferable therefore that “Priests who are present at a Eucharistic Celebration, unless excused for a good reason, should as a rule exercise the office proper to their Order and thus take part as concelebrants, wearing the sacred vestments. Otherwise, they wear their proper choir dress or a surplice over a cassock”.218 It is not fitting, except in rare and exceptional cases and with reasonable cause, for them to participate at Mass, as regards to externals, in the manner of the lay faithful.
Like Sarabande, I have seen my share of huge diocesan liturgies where all of the priests are concelebrants. While they will sit in the pews (the sanctuary at the Cathedral is too small for seating that many priests), they will all go up and gather around the altar for the Liturgy of the Eucharist.