Your examples seem like cases that can easily be handled by a checking account.

> Fundamentally, I should not have to explain why I would desire immediate access to my own cash reserves at a moment's notice to excuse broken financial infrastructure.

Yes, if you are investing in less-liquid assets to get a better RoI then you absolutely do. If you don't like the terms then you are free to keep your cash in your checking account.

Personal and certified checks aren’t immediate or same day settlement, only wires or Zelle transfers are (Zelle transfers can take more than a day depending on the sending and receiving bank).

If the settlement isn’t rapid, you can’t guarantee the funds will actually settle (checks). Lots of situations where someone wants the same guarantee as cash or a wire.

Most people are reluctant to accept large personal checks for one-shot transactions. Most people selling cars on Craigslist won't just take a personal check, for instance. It depends a bit on how reputable you look. You can't write one as bail, or to get your car out of the impound lot.

Most people are reluctant to accept large certified checks or money orders, going back to at least the 90s, because of counterfeits.

Just bought a brand new car this week from a dealership (US) and the seller refused to accept a check.

They’ll generally accept a cashier’s check. Ours would also accept a personal one as long as you didn’t mind them delaying mailing the title for a few weeks.

I made a down payment on a car with a money order and the dealer yelled at me that the agreement said they weren't allowed. Eventually they relented, because they wanted to complete the deal.

Back in the 90s, I tried to pay for a computer with a certified check and CompUSA treated it as a likely counterfeit.

That’s crazy. I’ve never had that problem, even when buying cars out of state. Were you paying the full amount by check or something?

Yes, was attempting to pay full amount. Couldn't remember my PIN to unfreeze my credit, and just seemed like it would be easier since I was in a position to pay in full.

That makes somewhat more sense. Trading a signed title without a lien for a check places a lot of risk on the dealer. Although there are ways to mitigate this, maybe the dealer in your case didn’t feel like bothering or it didn’t occur to them.

I don't think the question is so much asking "why", but rather "what" does "access to my own ... reserves at a moment's notice" even mean in a debt-based financial system?

Writing a check is instantaneous, but you obviously don't mean that.

If you're talking about wires and the like, then what you're really paying for is the sending institution taking on liability if there turns out to be a cascading problem.

People buy cars in Europe using debit cards, at least in the UK.

I’ve bought one with a credit card in the US.