I also find it completely strange that here in the US people tend to use feet as a sub-division of miles, but the mile/foot relation is not trivial: 1 mile == 5280 feet. And why would you need so many units for length? mile, yard, feet, inch... All (apparently) unrelated.
The metric system on the other hand, is so elegant and simple. Only one unit: meter.
Growing up in the US, we're exposed to these units our entire lives. End result? When my GPS tells me to turn right in a quarter mile, I can eyeball that distance. If it told me to turn in 400 meters, it'd make me pause a bit.
When I buy a gallon of gas, I know what I can expect to get from it. I know roughly how much a gallon is and how much a liter is, but I'd have to do math to tell you how far my car will go on a liter of gas. Don't even ask me me how many liters I need to travel 100 km…
It doesn't help that gasoline is sold only by the gallon, fuel efficiency is given in miles per gallon, speed limits are in miles per hour, and mile markers give our position on our highways and number our exits. Non-SI units are burned into our brains and our infrastructure all over the place. Grow up in that world, and the awkward math doesn't seem all that bad compared to having to get your bearings in a whole new system.
(That's not to say we shouldn't be switching to metric… but there are definite downsides for a population so used to our strange way of doing things.)
Besides, these length units do have integer relationships with each other: 12 inches to a foot, 3 feet to a yard, and 1760 yards to a mile. I find the first two easy to remember, and the third easy enough to calculate by remembering that one mile is 5280 feet.
But a yard is 0.9144 meters, making a foot 30.48 cm and a mile ~1609 m. Math within metric is easy. Math within our silly units is slightly more complicated, but we get used to it. Conversion between the systems is messy.
It's worth saying that lots and lots of countries have gone through this conversion process, sometimes multiple times -- think of all the countries colonized by the British (and thus converted from their existing systems to the British measures), who then sensibly converted to metric at some point after independence.
All of them were equally wedded to the pre-change system (fixed in mental habits, etc.), of course; there's nothing special about the American situation.
There's obviously a cost, but it's most severe for the older generation (whose mental habits are so ingrained they have real trouble adjusting) -- even for younger adults, your brain adapts to the new system after using it regularly for a while. AFAICT (I moved from the US to a metric-using country 7 years ago) it's never quite the same as the system you grew up with, but I'm pretty comfortable with it now.
It's something like learning bad words in another language -- they never gain the burned-in power of expletives in your native language, but your ears certainly learn to perk up when you hear them, and you can use them perfectly well.
Summing up -- quit whining, America, switch to better measurement systems, and the difficulties will smooth out more quickly than you think. :)
The metric system on the other hand, is so elegant and simple. Only one unit: meter.