1. Live chat support.
2. Phone support.
3. If something goes wrong, customer relies on you only (or your wife/aunt/uncle/grandpa at best).
4. You screw up the paper? (not necessarily because of your own fault) Customer relies on you only, no other options, time wasted.
If an order is small it's possible to manage a crisis. What about a dissertation, PhD thesis, or any other long-term project? A single point of failure (you, a single writer) and months or work can be lost (I don't know many writers who would want to continue writing a dissertation not started / researched by them, not to mention trying to keep the same writing style - the cost would be more than starting it from scratch). At best you can refund, but you won't cover the cost of having another writer start it from scratch to meet the deadline (reputable services would).
So in reality, a customer has two choices:
1. Order from Amazon and have all the conveniences and protections they offer. Have a piece of mind that third-parties watch the progress or deadline and act if something goes wrong.
2. Order from an ebay store and deal with an owner by email.
Price - about the same. If given a choice, students would likely choose Amazon, wouldn't they.