i suppose surly doesn't offer lifetime warranties because
they don't need to. steel frames don't break. they build
a quality product at a reasonable price. offering a
longer warranty would imply their frames break.

other bikes offer lifetime warranties maybe because they
have to. perhaps people consider the frame material or
the manufacturer substandard, and aren't willing to shell
out that much money because of higher potential failure
rates. but the warranty isn't free, it's just added into
the already too high price.