Why all German homes roof fixed with tiles?

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Because Germans are known for building things with materials that last.

Not all German houses are.

Properly constructed tile roofs are as warm as any other kind.

If you think that's bad you should see Scandinavia, it's all tiles and even further north! See also Iceland.  It's a good, long-lasting building material requiring little maintenance.  (Actually, a lot of those tiles are cast cement these days, the terracotta ones are dearer).  Think of the tiles as the outer, waterproofed layer.  Its job is to keep everything else beneath it dry, including the insulation.

insulation solves that 'too cool' possibility.  tile roof lasts a lot longer than American style shingles.  -- grampa

the roof does not have have much to do with how cold a house providing the ceilings are well insulated ..here in uk the standard is 11" of fibreglass ..Germany probably has more 

Most British houses are and it's also cold. A lot of buildings were thrown up quickly and cheaply because of the war and there are still many in use, new builds have slates to maximise profits. You can also insulate your loft with fibreglass and I think most people have, or had it done by their landlord.