Why is electric force and the magnetic force considered to be one electromagnetic force?

Classically, Maxwell's four equations that describe electric and magnetic behaviour show that the two are interlinked and you cannot fully describe the behaviour of one force without including the behaviour of the other.

In limited situations such as most electronics, we are are not interested in the full general case behaviour and we can work with easier restricted rules. But nevertheless, the full behaviour requires both forces and classically we still consider them as two forces.

In modern quantum / particle physics it turns out that the two classical forces can be explained as deriving from a single aspect of nature (along with the weak nuclear force) but the details of how that works are beyond me.

Electric fields and magnetic fields *can* exist on their own as long as they are static. Changing or moving electric fields induce magnetic fields, and vice-versa.

Magnetism can induce an electrical current and vice versa.