Saigon and Hanoi are on flat coastal plains too, but you don't decisively win a war by only controlling part of the country, and Myanmar's economy is propped up by gas reserves and other resources in the wild northeast. Myanmar's army is less likely to inspire devotion than North Vietnam's, but can easily be propped up by China if it fancies a proxy war. The likely result of conflict would be everybody being a loser.
The same is much with the rest of South Myanmar, where most of country's population, and economy is.