Psalm 89:47 Remember how short my time is: wherefore hast thou made all men in vain?

The above question/statement is rhetorical, our time is short, but why? And men are vain, but again why?

Our time is short because:

Romans 3:23 For all have sinned, and come short of the glory of God;
Job 20:5 That the triumphing of the wicked is short....
Proverbs 10:27 .... the years of the wicked shall be shortened.

The Lord will not allow the wicked long life, not long as we see (we think 100 years is long) but as God sees long = eternal. When Adam sinned, immediately, access to eternal life (God himself) was cut off.

As to why men are vain:

When Adam chose to sin, the result was separation from God. And anything we endeavor to do, separate from God, the Bible calls vain:

Psalm 127:1 Except the Lord build the house, they labour in vain that build it: except the Lord keep the city, the watchman waketh but in vain.

Only in Christ can we address both these issues. That is, only in/through him can we live long (eternally)
And to labour without him, is to labour in vain.
Philippians 2:16 Holding forth the word of life; that I may rejoice in the day of Christ, that I have not run in vain, neither laboured in vain.

Outside of Jesus, our lives are short and vain! Selah.