Tatmadaw isn't your typical military. Think of it more as a mafia organization. The generals, ex-generals, and their families and friends own all major conglomerates in Myanmar. They're involved in mining, hydroelectric power, fishing, rails, etc. All their officers are of the Bama ethnic group, who make up the majority of the country. Most of the soldiers are Bama as well, but they also use forced conscription (slavery) from all their other ethnic groups.

When Tatmadaw are engaged with Kachin rebel fighters for example, they'll use soldiers from other ethnic groups (Karen, Mon, Arkanese, etc) to fight them. When foreign investment started pouring into the Kachin area, Tatmadaw forced their way in by bombing civilian areas indiscriminately, causing mass displacement of the Kachin people. This allowed Tatmadaw to move in and take over those economic ventures. Their main driver is greed but their secondary mission is ethnic cleansing of all their minority groups.