The reality is that the pi isn't a general educational product - it's a computing educational product. And that is fine. If a kid wants to learn about programming, about what goes on under the hood, it's a fine addition to their locked down school Chromebook.
But you can do that on a regular laptop/computer running Linux sine that's what RPi also runs. You don't need an additional RPi to learn what's under the hood of a device running Linux.
And that gets really, really complicated when you have 1 teacher and 200 students to teach that week, each with a different hardware configuration, differing skill levels, devices in various states of repair, etc.
RPi is pretty much the cheapest way to get a brand new, standardized device into the hands of each of 200 students, then give the same curriculum to each student that will work the same, with predictable errors the teacher is going to be more ready to troubleshoot, instead of trying to figure out why some kid's PC segfaults because of an error on a Broadcom device that was only available on devices manufactured in May of 2014 (but sold in September of 2017, the ones manufactured 05/2014 but sold before 9/2017 are perfectly fine) when he tries to use any function using sin in Python (but the libc one works just fine).
Schools have historically paid a premium for this, like the Basic Stamp, which was really just a PIC microcontroller preflashed with some other stuff on the PCB to help it run, and sold at a crazy markup, but was well integrated into a standard curriculum that gave every student the same starting point and something that a teacher could easily help hundreds of students with. It's also kind of why Arduino became so popular -- did the same thing, but a fraction of the cost.
One good reason is that you can try stuff that would screw your laptop. Whereas with a pi it's just flashing the card again.
I used to work for a company that sold pi kits for schools, you could let 7-9 year olds try stuff that teachers and parents would not let them do to a laptop. Totally breaking a computer and fixing it again is a great experience, but not one you want to have on your primary machine
The reality is that the pi isn't a general educational product - it's a computing educational product. And that is fine. If a kid wants to learn about programming, about what goes on under the hood, it's a fine addition to their locked down school Chromebook.