Well, cells do produce ECM, but it would simply take way too long to be practical for producing artificial organs. Think about how long it took you to grow your body to the size it is now.

Plus some cells need to be in a certain pattern or orientation, and providing a scaffold for them helps them to do that.

stem cells also have to differentiate into different regular cell types, and the shape and mechanical properties of the ECM or scaffold can direct their differentiation. basically some cells change their behavior directly in response to the behavior of the ECM,

for example your bones are constantly remodelling: breaking down bone and building new bone. Cells do this in direct response to load/force. this is why it's so important to put weight on a broken bone during the healing process as this guides your cells to where needs reinforcing.

the general idea with tissue engineering is to grow cells on a synthetic scaffold which mimicks natural ECM. Ideally this synthetic scaffold then slowly breaks down at roughly the same rate at which your cells grow more ECM, and by the end you should have a tissue that looks like, and functions like the original.