At old times, just about all processors needed a high voltage for programming, since you needed energized electrons.

But with the advent of In Application Programming (IAP) where the running software may rewrite sectors of the flash, the chip manufacturers had to add a voltage doubler inside the chip to create the needed voltage for erase and write of the flash memory. This has allowed manufacturers to remove the need for external high-voltage modes.

As already noted, it's the support costs that makes the chip manufacturers protect their external programming algorithms. There are just too many people who wants to make their own programmers. And too many of these people also thinks that if they have a working programmer for chip A, then it is ok to program any chip with the same pinout with this programmer - or at least any chip from the same manufacturer. In short - the chip manufacturers do not want hobbyists or professional fools to try to implement their own programmers. Besides the high support costs, they will get a lot of bad reputation when the hobbyists then posts rude (and highly incorrect) comments about non-working (fried or just not correctly programmed) chips.