This fits the definition of a liquid rather than a solid. So, even though glass isn't flowing at a rate that you can see, it is a liquid rather than a solid. Certainly glass will flow faster if it is hot, and when obsidian is erupted from a volcano it is indeed hot. It still isn't able to flow very easily but it can at least flow a little. Because it is so viscous even when it is hot, it doesn't flow very fast or very far, nor can it spread out very much.
Obsidian flows are usually only a few km long at most, and are often tens of meters high.