Our Gemara on Amud Beis discusses why a career gambler is not trusted as a witness:
Rav Sheshes says: Those who play dice are disqualified because they are not involved in settling the world, i.e., in productive occupations that demand hard work.
What is this flaw exactly, and why does it disqualify them? Rashi here offers two factors which, combined, lead to a person who cannot be trusted to testify on financial matters because his perception is altered from a lack of appreciation for its gravity:
People whose sole occupation is gambling are not engaged in anything socially useful. As a result, they are unacquainted with basic business law and commerce, and have no fear of God, making them less averse to illegal activity.
It is easy to understand Rashi’s first point, that the lack of connection to the everyday life of contributing to the world leads to a mentality disconnected from reality, making their perception and judgment suspect. However, how does this cause disconnection from God? Perhaps being removed from mundane concerns should allow for greater spiritual contemplation and devotion. I believe the idea is that just as a child never truly appreciates his parents' efforts and sacrifices until he becomes a parent, so too, a person cannot appreciate the Creator’s kindness and goodness until he behaves as a creator.
The Rambam, in his commentary on the Mishna (Sanhedrin 3:3), offers a similar explanation:
Since the professional gambler is busy with something that does not benefit the settlement of the world or the foundations of the Torah, he is disqualified. It is not fitting for a person to be busy in this world except with one of two things: either with the Torah so that his soul may be completed with its wisdom, or with a work that will contribute to the maintenance of the world and society, such as crafts, repairs, construction, or merchandising. Of course, it is fitting to do little of these and much in the Torah, as the sages said (Avos 4:10): "Let him do little in business and be busy in the Torah."
The Rambam adds a concern regarding this person’s character: He is not contributing to the world through the acquisition of wisdom or through work that creates, repairs, or helps the economy by distributing goods and materials.
What would our sages say about a day trader or investment banker? According to Rashi, such a person certainly understands money and must work for it by researching and analyzing worthwhile investments and trends. This ought to instill a sense of humility, fear of God, and a connection to reality sufficient to be part of the social contract and to be trusted as a witness. However, the Rambam might find this still deficient. Investing on the scale of stocks or commodities is not like true commerce or personal investing in businesses. In the latter, the economy is being boosted, and actual goods and services are being distributed. In the former, where money is invested and divested in short-term trades, it resembles gambling on temporary ballooning and inflation of value and does not represent true exchange and trade, which promotes societal welfare.
Of course, in the final analysis, almost any trade or profession can be used as a force for good with the right intentions or can be ruined when approached without humility or fear of God. Even so, if one is choosing a career to begin with, it is worth considering the moral stance that the Rambam takes; not all careers are the same.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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