Why don't igneous rocks form fossils?

Igneous rocks are formed from magma and were - at one time - hot enough to melt, then they often moved all over the place, once molten. Any fossil would have burned away or disintegrated by the movement. Organisms do not normally survive intact because of the intense temperatures.

Fossils are most unlikely to be found in igneous rocks because they cannot survive through the process of formation of magma.

Most fossils are preserved plant or animal bodies living in or near water. There are no animals or plants which can live in molten magma so you will never find them in cooled magma, which is an igneous rock. There are however, exceptions. For example, where a lava flow covers a tree. The tree is burnt but the impression of it remains in the rock.