Because the judgment of the saints must also
include all the Christians who will die in the
tribulation, it can’t happen until after the
tribulation, at the 2nd coming:

Mark 13:24b-27
After that tribulation... then shall he send his
angels, and shall gather together his elect from
the four winds, from the uttermost part of the
earth to the uttermost part of heaven.

Ps 50:4
He shall call to the heavens from above, and to
the earth, that he may judge his people.

2 Cor. 5:10
For we must all appear before the judgment seat
of Christ; that every one may receive the things
done in his body, according to that he hath done,
whether it be good or bad.

Millions of saints will approach the throne to
be judged individually at the same time, just as
millions pray individually to Jesus and receive
answers from Him all at the same time today:

2 Pet 3:8
Beloved, be not ignorant of this one thing, that
one day is with the Lord as a thousand years...