The Myconids are network of semi-independent but fully sentient beings. They are one, and they are many.
Whether they cannot or simply do not want to, Myconids do not invade other dominions nor do their stationary disposition allow them to send expeditions. Fruitbodies are perhaps their most important member of society, providing a constant supply of new land.
Their stronger members of society serve different purposes, not least of all is fending off invaders.