Do you know what the incumbent re-election rate is in NZ or the UK? If it's 90% I'll certainly concede that it's not a uniquely American problem.
But it's a problem just the same. Along with federalism, checks and balances and the Bill of Rights competitive elections are part of the constitutional package designed to protect our liberties, and eliminating them from the mix bodes ill for the health of our republic in the long term.
In all "safe" seats the incumbant re-election rate is 100% (unless s/he retires). It is a different story in marginal seats - it depends on how the governing party is doing. But since marginal seats constitute only about 30% of all parliament seats assuming 60% of marginal seats are at risk this means the re-election rate among Westminster systems is typically around 82%. (But more typically only 1/5 of all seats are marginal. If that's the case then re-election rate is around 88%)