Pounds have a nice granularity for every day stuff: a 3 pound laptop and a 2 pound laptop are significantly different, but both are “about a kilo”.
On the other end, a gram is such a small amount that I can only perceive it in multiples, whereas I have a pretty good idea of how much an ounce is.
But oh, if you're willing to say "and a half" you might as well be willing to say .7 kilo.
There's no real overall advantage, you just picked a couple very specific numbers. You can always pick very specific numbers to make any system look better/worse.
Speaking of which, there's nothing stopping us from using something like SI prefixes with customary units, for example kilofeet. We just don't need to.