You are quite mistaken. Members of the cabinet, such a Cabinet Ministers and Prime Ministers, exercise in addition to legislative power, executive power.

Executive power is the setting of policies and directing departmental budgets.

Furthermore, they are privy to confidential information, which could be of value in commercial and market transactions.

For example, if someone knew that the government has a plan to invest in a particular area, and had bought real estate there in advance at a lower price with this knowledge - or alternatively if they influenced policy to serve a region where they owned real estate.

If they knew that a decision would affect the stock price of a certain industry and shorted their stock position - also known as insider trading.

However, if their assets are in a "blind trust", then presumably all their investments are at an arm's length - and they do not know where their assets are invested and cannot influence the investment.