PiHole exists precisely because the Pi is a general-purpose computing device: it uses a relatively stock operating system and network stack to perform its task. Its competition here is not a desktop computer, it's a bespoke network appliance with custom hardware/firmware manufactured by somebody like Cisco.
What you lament is that so many of our "PC" systems seem to have wasted processing potential, but I think this is optimizing along the wrong axis. Sure, my desktop PC could run a home DNS server without breaking a sweat, but my desktop PC is not on all the time. Even the idle power draw from the ATX power supply likely outweighs the full-throttle power usage of a Pi-equivalent device.
Home computing (PiHoles, routers, and "smart home" devices) are a devolution of computing to commodity status in the same way that the PC was a devolution away from a mainframe-and-terminal environment. I think further progression along this axis is inevitable, and we should welcome entries like the Pi or this VoCore that keep the ethic of hackable, general-purpose computing even in a commodity footprint.