Why is the Earth's core so hot?

It is due to several reasons.

1- it is the heat left over from the Earth's formation, due to the energy of the planetoids that aggregated to form our planet.

2- it is the heat due to tidal effect, as the Earth spins, the gradient of gravity caused by the Sun and the Moon knead the planet, causing some heat to develop (and it does have a side effect of progressively making the spin to get slower, energy is thus transferred from kinetic to thermal)

3- it is the heat due to the nuclear disintegration of radio-active elements, like uranium, that is present in greater percentage in the Earth's core than it is in the Earth's crust, due the those elements having a higher density than most, which makes them have a tendency to "sink". The Earth's core is a giant nuclear reactor.

Most other planets do have hot cores as well.

The main underlying reason for the heat is gravity, in several different manifestations. Mostly now though the heat is due to gravity compressing the rock toward the center. Heat and pressure are different aspects of the same thing, energy. The original formation of the earth was at first due to collisions of matter adrift around the sun. As the body grew bigger the collisions were less be Chance and more due to gravity. Pressure at depth started to increase and because gravity hasn't let up the compression continues and heat continues. There is still residual heat from the original formation as well(gravity-caused).

Some heat come from radioactive decay.

Some comes from gravity braking between the moon and the earth and the frictional heat this causes. Gravity again.

Planet big enough to become spherical have or at least had hotter cores. Smaller planetoids do not. AT some point the heat caused by gravity can be small enough so that heat is radiated out of the body and not accumulated, a clumsy way of saying the planet is cooling off, even to the core.

There are four reasons:

- The earth was composed of relatively hot material to start with

-- As the earth condensed from a mixture of rocky and metallic material, it compressed under gravity, producing a great deal of heat as it compressed, and the heavier metallic material gravitated towards the centre. This was the main cause of the high temperatures at the core.

- Heat is also produced from natural decay of radiactive elements (mainly uranium and thorium)

- The outer layers (the mantle and crust) form a very effective insulating layer, allowing heat to escape only very slowly. However, the overall heat loss is very slightly greater than the additional heat produced by radioactive decay (especially as the radioactive elements are gradually being converted to non-radiactive elements). As a result, the earth is gradually cooling; but less than a degree every million years. Before the core cools and solidifies, the earth will be vaporised as the sun expands into a red giant in about another 4 billion years.

- Oh, and Vincent G's answer about the effect of tidal stretching. Make that FIVE reasons.

The heat released due to friction and pressure is substantial. Also, radioactive decay within the Earth continually releases heat.

Pressure and nuclear heat from decaying radioactive material. Some have said it's 'original heat' from formation, but I figure 4.6 billion years is time enough to dissipate THAT!

It was originally part of the sun which broke off. The surface has cooled down but not the middle.

due to the high amount of pressure. pressure and temperature are directly related

It's where the Devil resides.

due to gasses