Youre right, lightning does make glass, although it is not how most glass is made. For example, the glass in your windows was not formed by lightning. The lightning has to strike rock or sand to make glass, and the resulting glass is called fulgurite. The reason lightning can make glass is because lightning has a LOT of energy. Because the lightning has so much energy, it can quickly melt the rock or sand to a liquid. But the lightning strike lasts for such a short time that it cools down quickly and the quick cooling makes glass. Glass made in this way is fragile and can break, just like the glass you are used to seeing.

This is a great question and is not yet completely understood.First, I should mention that although lightning strikes can produce glass in sandy soil and in some rocks, the glass that forms are not large intricate pieces as dramatic as seen in the movie "Sweet Home Alabama". Instead the glass produced by lightning strikes are usually small tubular pieces, yet they are still pretty. The glass is formed in sandy soils that contain silica (one of the compounds we make glass from today) In the rocks, the silica content may be much smaller but it is expected other minerals from the rocks contribute to the glass matrix. The glass tubes formed in rocks is more rare than the glass formed in the sandy soil.Lightning strikes to the ground carry energy of ~ 10¹⁰ Joules. (1 Joule = 1 kg * m² * s^-2) This energy and the rapid temperature change that results is likely enough to melt the sand and cool it into the glass pieces. The duration of the lightning strike to the ground is also expected to play a role.