I always thought it's the agreed-upon explanation ;). I.e. Star Trek's post-scarcity society is enabled mostly thanks to the replicator technology. So food, clothing and basic construction materials are essentially free and unlimited. People do whatever they want - most civilians just go and live and further their passions; some do work for others (e.g. Sisko's dad and his restaurant), but they do this because they want to, not because they have to. Many join Starfleet, which basically offers people grand goals and adventure, and enforces structure and obedience based solely on non-monetary incentives.

Things like starships are pricey partially because they're huge (and thus reaching the limit of civilization's energy output) and partially because some things apparently just can't be replicated with their-era replicator technology.