Because every theorem has an inverse proposition, the inverse proposition is a true proposition, and the inverse proposition is not necessarily a true proposition, so not every theorem has an inverse theorem.
For example:
Is the case of the inverse theorem.
Theorem: in the triangle abc, if the sum of the squares of the two sides is equal to the square of the third side, the triangle is a right triangle.
Inverse proposition: if the triangle abc is a right triangle, then the sum of the squares of the three sides of the triangle is equal to the square of the third side.
Right, so it's the inverse theorem.
It is not the case of the inverse theorem.
Theorem: equal to vertex angle.
Inverse proposition: equal angles are opposite vertex angles.