Re: Why
It's not surprising that as the available ARM cores (and available SoC) organically improved for embedded (often set-top) and mobile devices, RPi took advantage to improve the user experience. But yes, they've ended up at the point where your "by default" performance is way overkill for tinkering and past "basic desktop" level.
For general computing, you're better off with a used uSFF Optiplex or ThinkCentre. Quad core i5-4xxx boxen are £70-100, can be stacked with lots of RAM and have proper SATA/PCIe, even M2 on newer models.
The Pi has an advantage in GPIO and the ecosystem/library/community support for it.
But if hardware tinkering what you're doing, then a Le Potato has the Cortex A53 cores used in the Pi3, is form-factor compatible with the 3B+ and costs £24 instead of £33 (at the cost of no onboard Wifi or Bluetooth - you're using ethernet or dongles). Libre have focussed on that hobbyist side (as well as embedded things like signage, as they've emphasised 4K support, a decent video stack, etc). Coming in 30% cheaper is nothing to sneeze at.
That original educational idea that Pis were cheap and easy for schools to use for teaching (and hosing your install could be easily remedied by reflashing the SD card) never really seemed to pan out. If kids manage to hose a Windows install on a regular desktop whilst you're teaching them Python, then their user accounts have all sorts of permissions they shouldn't! You still need all the peripherals and swapping from normal PCs to RPis at the start of class is a faff.
As others have mentioned, an awful lot of the interesting hardware stuff is now going on around the Pico which gives you a super cheap microcontroller that you can easily stuff all sorts of logic into with MicroPython, without a full linux stack (and lots of community support).