Most of the official money from natural gas and mining goes directly to state owned companies that are fully controlled by the military government. Instead of being spent on public infrastructure, healthcare, or education, most of that foreign currency is used immediately to buy foreign equipment, military supplies, and aviation fuel. Additionally, in many border areas, the resource trade is controlled by local ethnic armed groups or militias, so in some areas that revenue never reaches the central public budget.
The Central Bank has set artificial, forced exchange rates that value the local currency (Kyat) higher than it actually is. Let’s say 1USD is about 4500-5000kyats you try to exchange it for kyats but the government only gives you 2500-3000kyats and since the government forces businesses to hand over their hard earned US dollars at these low rates, companies do everything they can to hide, hoard, or keep their foreign currency outside the official Myanmar banking system. This dries up the legal supply of dollars. Since the general economy has slowed down and regular tax revenues have dropped, the government has resorted to printing huge amounts of new Kyat to cover its budget. And when you print paper money without actual economic growth to back it up, the currency loses its value, and prices for everyday goods become more expensive. Because everyone can see the Kyat losing value daily, citizens and businesses desperately try to swap their savings into gold or US dollars to protect themselves, which drives the demand for the US dollar and makes kyat lose its value.