Wood decays when there is sufficient air and moisture. Wood used for construction is dried before being used, and unless there's a leak / excessive humidity, should be able to last a very long time without substantive decay. Wood in stuff like furniture is also often sealed with a specific wax/oil layer to keep out moisture, further preventing rot.

Theoretically it will decompose someday, but at the point where we have significantly less buildings I assume either other bad things have made this less of a concern, or we'll be advanced enough to recapture that carbon in other ways.

...but yes, if you're willing to "waste" the resources, just thoroughly burying wood away from air should also work.