AIUI, there's two problems here. First is the fact that many buyers (particularly first-time buyers, who are more likely to fret over price) don't really know what size bike they need. That's a very good reason to deal with an LBS, who should work with you trying out different bikes and bike sizes to find what you want. If you order on-line, the onus is all on you.

Second, bikes that look identical on paper from the specs often don't feel identical. One of the best bits of advice is to buy what you like to ride, because then you're more likely to ride it. Again, if you buy after looking at a piece of paper or a web browser, the onus is all on you.

If you want to increase the odds of getting a bike that feels right, fits right, rides right, and that you're excited to ride; and at the same time get someone to share the blame if the above don't happen; then you might want to buy from your LBS.