Because there is an objectively better option out there already, which USB-A does not have any benefits above it (other than wider adoption, which is always going to be the case for an older standard at first).
Except nobody is making that case because Ethernet does not have a better alternative. There is no "Ethernet Type-C" (to call it something) that improves on the task of providing wired network access. USB-A does.
It's not immediate, but the benefit is in forcing peripheral manufacturers to adapt to modern technologies. USB-C has been out for almost 10 years now, is just as cheap to implement as Type-A is, and benefits from reversibility, more reliable connections, taking less space in a computer, etc.. Why are we still making stuff with USB-A? Because computers still come out with the port. We'd still be using floppy disks if computers could still read them natively.
I was asking for this since before Apple made this choice. Hell, we don't even know if it is in fact a choice Apple made because the machine hasn't even been announced. But people just like to chalk up anything they disagree with as "fanboyism". Par for the course around here.
USB-A has a number of benefits already mentioned in this thread, USB-C is not objectively better for every purpose USB-A is used for.
You seem to have missed the point on the ethernet comparison, ethernet does have a better alternative for the average end user - it's called wifi. I'm a fan of wired connections but the average person does not use them, and if making a case for phasing out a port - that's the best case there is, lack of use. USB-A is widely used and thus lacking a strong case for removing it.
Things like floppies, dvds and cds all had very linear product cycles, ports are different - peripherals like a mouse, monitor or keyboard all remain useful for much longer than rapidly evolving storage capacities. A mouse and keyboard don't benefit from USB-C, there's nothing ground-breaking there.
Devices have been produced for years now with USB-C, but not everyone buys new devices and those who do don't all buy newer peripherals. Forcing peripheral manufacturers to adapt technologies consumers don't want isn't beneficial. This isn't how the free market works, when people stop buying USB-A products, the market will shift naturally. Also, USB-A is cheaper than USB-C, what makes you think USB-C is just as cheap? Furthermore, USB-C tends to be physically more frail than USB-A, so I don't know where you have the idea that connections are more reliable - that's just not the case, USB-A is very reliable and resilient.
USB-A isn't obsolete and it won't be for awhile, it's more than capable for much of what the average user needs it for. Let's not forget we are talking about a desktop, DESKTOP, absolutely no reason to kill USB-A on a desktop. I'm sure as people's USB-A peripherals fail or if they actually want a newer one, there will be less need for USB-A, but forcing this is just senseless.