Have you ever seen people on Xbox Live? I guarantee you, unless there was something stupid like a "1 million miles" or "1 million races" achievement, everything else - everything - would be unlocked by most regular players within the space of a few days. The ones that are easy to get (like a lap on every track or drive every car), everyone will have. And the ones that are hard to get (beating a certain lap time or completing a large number of laps) will only be accomplished by the people with already-notable LFSW achievements.
So the folks like me who can't beat the laptime will get frustrated and either ignore the achievement entirely (making it pointless) or move offline or to a private server to attempt it (meaning a reduction in server population). And the people who can do it no trouble will get the achievement in no time, putting them right back where they'd be now - with the achievement already listed on LFSW.
Pointless false replay value and not worth the time it'd take to code it. NOW you're done here
So the folks like me who can't beat the laptime will get frustrated and either ignore the achievement entirely (making it pointless) or move offline or to a private server to attempt it (meaning a reduction in server population). And the people who can do it no trouble will get the achievement in no time, putting them right back where they'd be now - with the achievement already listed on LFSW.
Pointless false replay value and not worth the time it'd take to code it. NOW you're done here