Why is Earth's entire mantle not liquid?

If we were to start making a hole in our planet, we would encounter different layers of our planet. The topmost layer of Earth is known as the crust (5-25 miles deep). This is the part on which we live. Next, we will encounter the mantle, a layer which is about 1800 miles thick. If we continue digging, we will encounter the outer core (1400 miles thick), which is then followed by the inner core (800 miles thick).
Earth's mantle is composed of very hot, dense rocks. All these rocks should ideally be liquid, given the high temperatures (1600-4000 degrees Fahrenheit) in that region. However, the increased pressure in this region increases the melting point of the rocks. This ensures that even though many of the rocks are very hot, they never reach their melting points. This results in the mantle being made up of mostly solid rocks. In other words, Earth's mantle is not completely liquid owing to the high pressure in that region.
Hope this helps.