you pay more because the shops have to maintain MAP pricing. what is unfair is that while a shop is required to hold a price, unauthorized internet retailers are not.
install should be about $35/hr for basic stuff and no more than $60/hr for elite installer time. that's cheaper than a mechanic.
it takes time to do a job correctly. if someone says they can install an amp in an hour, tell them "no thanks". something that fast and cheap will require cutting corners. paying a shop a few hundred for a good amp install, is fair. paying a shop a few hundred for component speaker install is fair.
you are always welcome to DIY, but it requires a place to do it, plenty of tools, and a combination of general and specific knowledge about electronics and vehicle maintenance. taking a car apart is trivial for some and monumental for others. for most of the people i see posting questions on their DIY work, they spend more money replacing gear and redoing work that was done wrong than if they just paid a competent shop.
the biggest challenge is determining who is competent and making sure they actually care about their work. most installers can do a good job, it's a matter of their attitude. i recommend anyone looking for an install spend some time talking to the installer about the car, discussing the work and expectations. if the installer won't talk to you, don't let them touch their car. that conversation will give the installer a reason to do a better job - just remember to treat him with respect and understand he may have more experience than you do. also, ask to see work in progress - i can usually talk my way into the install bay and see jobs they are already doing.