There are probably various reasons as to why people do not trust in God but trust in themselves or in the world. I suspect some people are defiant and want what they want, some are greedy and want more and more, some are envious and want what you have, and some are spiritually hollow and live in a constant state of want no matter how much they accumulate. Then, there are some people who are traumatized from past betrayals and who fear to trust anyone ever again — including God.
St. John of Kronstadt wrote about trusting in God, giving oneself up to Him, rather than focusing on the acquisition of wealth and finding happiness in the things money can buy. I divided the following quotation into paragraphs to make it easier to read.
Give yourself up entirely to God’s providence, to the Lord’s Will, and do not grieve at losing anything material, nor in general at the loss of visible things; do not rejoice at gain, but let your only and constant joy be to win the Lord Himself. Trust entirely in Him: He knows how to lead you safely through this present life, and to bring you to Himself — into His eternal Kingdom.
From want of trust in God’s providence many and great afflictions proceed: despondency, murmurings, envy, avarice, love of money or the passion for amassing money and property in general, so that it may last for many years, in order to eat, drink, sleep and enjoy; from want of trust in God’s providence proceed in particular afflictions such as arise, for instance: from some loss of income through our own oversight, from the loss of objects, specially valuable and necessary, as well as immoderate joy at recovering some objects, or at receiving some large income or gain, or some profitable place or employment.
We, as Christians, as “fellow citizens with the Saints and of the household of God: (Ephesians 2: 19), ought to commit all our life, together with all its sorrows, sickness, griefs, joys, scarcities and abundance unto Christ our God.
St. John of Kronstadt
My Life in Christ, pp. 251, 252
In general, as a lifestyle or emphasis, amassing money and possessions is contrary to trusting in God. That is, trusting in oneself and focusing on the ways of the world rather than turning to the God Who created us. We might say that we find happiness in whatever it is that we trust in — except that trust in self and world is misapplied and temporal, and therefore ultimately disappointing and even ruinous. St. John connects lack of trust in God to various forms of distress and affliction — I think not as a punishment from God but as the natural progression of that which is prone to instability and deterioration.
St. John reminds us that we belong to the household of God with all the saints. It follows, then, to give ourselves up to God, to do His will which can only benefit us, and to commit any needs or abundance into the service of Christ in Whom we live and have our being. Thus, whatever our financial status, whatever our health, whatever the betrayals of the past, we are restored and fulfilled only in Christ Who is worthy of trust and Who offers us the Eternal Kingdom. There is nothing else to want and no other true happiness.