The same reason I change the engine oil after a 50 mile break-in. I don't care how clean you think the engine is during rebuilding, there is trash hidden in it somewhere. Getting backto the lifter issue, years ago I bought a new set of lifters, and one was bad. I just changed the one and that's been 35 years ago. Sometimes people over think. Just a thought, is it possible the lifter bore is over bore.