You are right that snow is a good insulator. You are wrong in the idea that being a good insulator is the only requirement for a warm base. Snow had a lot of air in it, so heat doesn’t transfer through it well. But snow is also literally freezing cold. Since it’s also water, it can absorb a lot of heat quickly. I mean come on, everyone knows their hands get cold when they touch it. Their feet get cold when they stand around in it, even in insulated boots. Snow is consistently and relentlessly cold.

Snow shelters work well at keeping things relatively warm, but that’s only relative to the outside temperature, which can be significantly colder. They will never get much above 32F inside. Any heat in the air inside will quickly be absorbed into the snow. Beside that, a snow shelter should have a vent in it, which has the consequence of allows warm air to escape very efficiently. It’s still better than a tent, but it’s by no means warm.

When I was a kid, I believed the whole “snow is a great insulator” bit too. Twice I slept in snow caves with a closed cell foam pad under my sleeping bag. I was cold, but I got through. Now that I’m an adult with real experience, I understand it better. Most people (myself included) use a closed cell foam pad plus an Inflatable pad with an r value of at least 4. This is pretty standard among mountaineers, ski tourists, and winter backpackers. If you have any less insulation under you, you will probably feel the cold of the snow below you. If you feel it, you are losing heat to it.

No snobby source required.