Answer:
Due to the iron inside the earth.
Step-by-step explanation:
Imagine a sphere of Iron, as big as two-third the size of the moon and as hot as 5700 Kelvin. That is the Earth’s core.
The iron core isn't in its liquid form even at that temperature because it is crushed under immense gravity. This core is surrounded by 2000 km of other metals like iron and nickel which are in their molten state.
The temperature is not the same at every point in this molten layer. The hotter and less dense matter rises up, and the warm denser matter sinks. This causes convectional currents in the interior of the Earth.
Because of the Earth’s spin, there is a force that is established called the Coriolis force which causes swirling whirlpools here too.
This flow of molten metals produces electric currents which generate self-sustaining magnetic fields. And as a result of the Coriolis force, all these combined effects add up to produce one big magnetic field engulfing the Earth aligned in one direction.