They which pierced Him are long dead, how could they see Him descend with the clouds?
...here is what Gaebelein says (and Gill, quoted below that, agrees),
[quoting]
"[The Book of] John alone mentions this blessed fact. ['pierced'] “It is a beautiful testimony of divine grace, answering the last insult man could heap upon Him. They drove Him outside the camp, put Him to death on the Cross, and then, to make His death doubly sure, the soldier pierced His side. Salvation was God’s answer to man’s insult, for the blood and water were the signs of it.” John speaks of this never to be forgotten occurrence, in his first Epistle (5:6). There he mentions water first. It denotes purifying which man needs, and that has come with all its attending blessings by His precious blood. But notice John writes: “And again another scripture saith, They shall look on Him whom they pierced.” He does not say, another Scripture was fulfilled. Zechariah 12:10 was not fulfilled when He died, but will be fulfilled when He comes again and the believing remnant of Israel mourns for Him." --Gaebelein, Commentary on John
[and quoting Gill's Commentary on John]
"they shall look on him whom they pierced; in the Hebrew text it is, "upon me whom they have pierced"; the reason of this difference is, because Christ, who is Jehovah, is there speaking prophetically of himself, here the evangelist cites it as fulfilled in him, that is, that part of it which regards the piercing of him; for that of the Jews looking upon him and mourning is yet to be fulfilled, and will be at the time of their conversion in the latter day, and at the day of judgment. And as the piercing of the Messiah has been literally fulfilled in Jesus, there is reason to believe, though the Jews are to this day hardened against him, that that part of the prophecy which concerns their looking to him, and mourning for him on account of his being pierced by them, will also, in God's own time, be fulfilled. Nor is it any objection to the application of this prophecy to our Lord Jesus, that not the Jews, but the Roman soldiers pierced him, since what one does by another, he may be said to do himself: though it was a Roman soldier that pierced the side of Christ, the Jews might desire and urge him to do it; and however, they agreed to it, and were well pleased with it; and just so Christ is said to be crucified and slain by them; though this was done by the above soldiers, because they prevailed upon Pilate to pass the sentence of death upon him, and to deliver him to the soldiers to be crucified." --Gill, Commentary on John
[end quoting; brackets, bold and underline mine; source: Bible Hub]