True but playing devil's advocate you can also expand the storage via Thunderbolt 3 which is around the same performance as internal. For a dekstop that doesnt move around I dont see the argument why not.
There's nothing that says you cant buy the base storage and word exclusively off external SSDs. As others point out that sometimes is better for data loss prevention having it decoupled from the OS.
A laptop, ok, one could debate upgradability there to not carry around another drive. It's still a weaker argument when you're carrying around a case of cables and a docks though personally; and for the types that need that much storage likely are already carrying a bunch of "stuff" anyway.
Im certainly not praising Apple here, but I dont see the huge deal either, especially on a desktop