Why are short journeys considered bad for your car?

[xr287]

I've never really understood this. Ok so it might not get warmed up properly so this might cause more wear because the oil isn't reducing friction as much as it might when hot? Or as components in the engine expand with heat they fit together better and so wear less?

Are either of these two ideas even right? Even if they are the same wear would occur while the engine is warming up when starting out on even the longest of journeys.

So why are short journeys bad for your car and manufacturers recommend more regular services for cars that are only used for short journeys?