Because every carrier is looking to make a profit from stocking/marketing/servicing the devices they sell. Google sells the phone because of LG; Google isn't a cellular carrier so when they sell the phone, they're not making much money off it at all because it's a) unlocked and b) unsubsidized.
Because carriers like Sprint and the others know the Nexus 5 (and all Nexus smartphones) are unlocked from the factory that means if they sell you a Nexus 5 at the same price you can typically get it for from Google directly, you're not going to have any real incentive to stay with them after the purchase since you can use it with any provider (more or less).
The only way a company like Sprint is going to make their typical profits or even close to it is marking up the price over what the manufacturer/designer is selling the device for hence the higher cost.
Does it suck when this happens? You're damned right it does, but that's business for you.
Just think of how many scumbucket bastards are out there that have been buying the Nexus 5 in somewhat large quantities since Friday only for the express purpose of putting them on eBay/craigslist and taking advantage of people by snatching up all the inventory then selling it back to you at a serious markup as there are literally thousands upon thousands of them.
I see ads on craigslist right now for the 32GB models for $550+ and the people selling them don't even have them for actual sale yet
because they're not going to get them for several days (after Nov 8th) - talk about bottom of the barrel crap, I swear.
If you want the device, and you have to get it "now" then carriers are the only option since it appears Google still has inventory but the shipping times are 3-4 weeks now, then you end up paying whatever someone else is charging.
Supply and demand, run by scumbuckets.
Currently in between devices...