ELI5: what's the meaning of the spread between bond and bund?

The yield on the US 10-year bond is at 4.5%, while that of Germany is at 2.5%. Even Italy's (bbb+) is at 3.8%.

What does this spread represent and why isn't there an opportunity for market makers to perform arbitrage?

Since I don't understand this mechanism, being European, for the "safe" part of my portfolio I always stick to European bonds...