>What's weird about it to me is that this is one of the few places in US consumer markets where the seller cares deeply about your method of purchase and where the money came from.

It's not weird at all. Buying a car or TV from Walmart with a credit card does not carry anywhere near the risk of the transaction failing that buying real estate does.

It's simply a function of the probability of the transaction succeeding (or "closing" as it's commonly referred to). With a loan, there are multiple parties whose requirements need to be met, from the lender, the home insurance, the title insurer, the seller, etc. The more entities you cut out, the less chance of the transaction failing. Underwriting a car is also much more simple and less risky for a lender than a house, which has much higher downside risk and unknowns.

Not to mention the myriad laws resulting in legal liability and opportunity costs relating to real estate purchases, as opposed to a car purchase or a TV purchase where the worst that can happen is the seller takes it back and sells it to someone else.