If Greece defaults, several major French banks will go under unless the French government steps in to save them.
If the French government does act it will massively increase their sovereign debt which, in turn, will result in the collapse of both their own credit-rating and that of the European bail-out fund. The result would be the collapse of world stock markets due to financial uncertainty, leading to a decade or more of world-wide recession (with millions of job losses).
If the French government lets their banks fail, it will lead to banks ceasing to lend to each other (as happened in 2008) due to the fear that other banks, in other countries, might also collapse. (I've cited France here as the country which might suffer most from a Greek default but many other countries, and their banks, would be affected as well). That would also lead to worldwide recession.
The world (and not just the EU or the Eurozone) simply can't afford to allow Greece to default. The risks are far too great.
Chris