Here's why:

* Dissolved gases escape as magma rises: When magma is deep underground, the pressure is immense, keeping gases dissolved within the molten rock. As magma rises towards the surface, the pressure decreases. This decrease in pressure causes the dissolved gases to expand and escape, forming bubbles within the magma.

* Volcanic eruptions: The escaping gases are what drive volcanic eruptions. The expanding bubbles create pressure that can eventually force the magma to erupt explosively.

Therefore, the dissolved gas content of magma *decreases* as it reaches the surface, not increases.