Many additions to cart, few purchases

That's an incredibly bold and generic statement to make without any basis of fact.

What if what you are selling costs $5,000 or more? Do you really think that 30-40% of people who add that amount to their shopping cart are simply going to check out? If so, you have never sold any high ticket items.

Very often - especially with high ticket websites - people are adding something to their cart and even hitting the checkout page (but not actually completing their checkout) because they want to see if there are any hidden surprises (shipping/taxes/handling fees etc,). They will then do the same thing with several other websites until they find the best deal with a website they are comfortable buying from.

With large purchases, there are often other people to consult (like your spouse) and major budgetary decisions involved. People often take weeks or more to make those decisions. Many keep revisiting and wishing they had the money, trying to justify spending it without ever pulling the trigger. Many will visit dozens or even hundreds of times over weeks/months/years and still never pull the trigger.

Then, of course, a whole lot of your conversion rate depends on where the traffic came from. Are they a repeat customer? Were they referred by a customer? Did they come from an ad? How well was the ad targeted? Or, did the traffic come from organic search and how targeted was that search (e.g. if you are selling bowling balls, was the search simply "bowling ball" or was it "blue and black Brunswick 16 pound bowling ball")?

Sorry, but your pronouncement may very well be valid for an item that sells for $6.99 and comes from a super targeted Facebook ad, but the greatest, most targeted Facebook ad in the world is not going to have those same conversion rates for something that costs $3,000. Even a $6.99 item will not have stellar conversion rates if the traffic comes from a super generic search.