Lots of reasons starting with privacy/security, lack of front end gear, not wanting the hassle of tire-kicking fruitcakes coming around, and simply wanting a quickly transacted sale. Your reservations are noted but it is an agreement between two parties that constitute successful sales.
Many spkrs have enough reviews and internet buzz that you pretty much know what you're getting before the fact. And how many dealers let you take valuable stuff home [and off the sales floor] for an audition? Sure, you can listen in the store with unfamiliar equipment in an environment almost guaranteed not to be anything like your own room, and with unfamiliar material if you didn't bring your music. Yes, lots of pitfalls in the spkr buying game. One recourse: after you buy, break them in, tweak and listen for awhile, you acclimate to the sound and often end up liking what you bought, albeit in a back door kind of way. Good thread.