ITT: a bunch of people commenting on $6000 bikes who have never ridden $6000 bikes.
OP: it's about optimization and design effort. It takes a lot more engineering effort to design a sub-15lb bike than it does a 17+lb bike (and the price reflects this as much as it does the raw material cost - $4000 buys about a week of one engineer's time; development cycles are in the years of man-hours). Two pounds might not seem like a lot, but 2lbs is 1% of a 200lb bike+rider system, which is directly 1% faster up a hill, which is 6 seconds on a 10 minute hill. Races have been won on far less margin.
If none of this seems important to you, then you are not in the market for a $6000 bike. Not everyone is.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter