It's economics 101. A skilled laborer in the US cannot survive making $10/hour. So, say they can squeak by on $15/hour but should be in the $20/hour range to really do well - as befitting their skill level in context of US society.
For the sake of easy math, let's say he can make one sweet guitar a week, the manufacturer is now shelling out about $900 in labor (his hourly rate, plus benefits, plus unemployment insurance, payroll insurance, etc.) for 1 guitar. They need to make a profit on that labor, so the labor portion of the wholesale price is now, $1500 assuming the company isn't uber greedy.
They buy parts in major bulk and let's say it costs $200/guitar for all the parts necessary (very conservative number, I'm sure, but just for the sake of the illustration) which they turn and add $400 to the price of the guitar.
You now have an $1900 guitar, wholesale, which the retailer is going to sell for, let's say $2500 so they can keep the lights on, pay their Eddie Van Halen wannabe to help you buy it and/or just stand around and pretend to look cool.
Do that same thing in Mexico, where a worker makes $10 A WEEK. and suddenly you've freed up $880 in cost, which allows you to wholesale the guitar for $500 and the local Guitar Center can sell it for $800 and hire even more guys to stand around and try and look cool while teenagers come in and butcher Enter Sandman on their lunch breaks.
The US skilled work isn't necessarily any more skilled than the Mexican one, but the cost of living and all that is hugely different.
(for reference, I've stood on the property of Mexican auto parts manufacturing plants where their employees are paid $100/week, then charged $90/week to live in the 8'x8' concrete box the company forces them to live in if they want to keep their job.)
Quality might not have squat to do with it, economics has everything to do with it.