Far more receivers are sold than dedicated amp/preamp combos. That's why receivers are a lot cheaper than amp/preamp combos. Also, manufacturers have long figured out that audiophools are willing to shell out big bucks for beefy looking audio equipment, so manufacturers really take advantage of this and charge big bucks for amp/preamp combos.
Technically, a receiver is far more complex to design and manufacture than an amp or a preamp. And a receiver represents a far better value for the dollar.
A mid range receiver should be more than good enough for most speakers. You only need an external amp when you are trying to drive 4ohm or esoteric 2ohm speakers.
A preamp is just a volume pot that has a bunch of connections that allow you hook up your CD player, DVD player, etc. to the amp. In the old days of record players, that signal coming from the record player is really low, so it has to be "pre-amplified" before it can be sent to the amplifier. But nowadays, signals from CD/DVD players are more than beefy enough to be fed directly to the amp. All the preamp really does these days is to attenutate the signal strength and thus adjust the volume. That's about it.