You're comparing a microcontroller (Arudino) with a computer (pi). They aren't the same thing.
I mean, sure, you and I know the difference between an 8-bit micro with some friendly libraries and a 64-bit ARM SOC running a full OS. But for a lot of stuff they accomplish the exact same thing. So why not compare them by what most people do with them rather than their raw technical specs.
..Unless you're talking about something like the Arduino Yun, which actually runs a Linux OS. But as an example, you're probably not going to build a wifi security camera on the original device.
All I'm trying to say is that for the vast majority of users a raspberry pi can do anything that an arduino can. Sure you might not be able to so some super tight real-time stuff that you could do with hand-rolled asm that you could do on an arduino, but at that point you're more at the level of 'using an AVR' than 'using Arduino'.
When the problem is "I want to control some RGB LEDs" or something, either can be used. That means that they are of the same category.
Furthermore, most people using Pis and Ardunios are hobbyists who are necessarily familiar with the raw technical implementation and are choosing the product based on that technical data.
OTOH, all the folks saying the knockoffs are $2-10, that's cheaper than a Pi.
Heck the last time I looked into doing sleeping on an AVR-based Arduino it basically said go look at the 328's manual to set the registers directly. (Although that was a long time ago.)
For non-hobbyist problems where you really need a microcontroller, it is selected by spec (in terms of performance, operational predictability, physical/material properties, power consumption), and then purchased in volume (~ millions). Employing a general-purpose CPU for said problem, while potentially alright when prototyping, would not be satisfactory.
Different form factors, and most importantly different energy need. You'll get much better result running an arudino on AA batteries than a pi.
I needed to control my brewery's heating element based on input from a timer and a thermometer. That definitely called for an arduino.
They're different, and while it's true I COULD have used a Pi for the latter, it would use way more power and be complete overkill.
But, the original comment was taking about the price difference between the arduino and an RPi. Sure the RPi is 'overkill' for for controlling brewing beer, but if it's cheaper, then why not use it? I could understand this argument if the more-feature-full thing was more expensive, but not when it costs more for less.
Don't get me wrong, I love Arduino. I use it way more than I use RPis. I just use it on $2 ESP8266 boards straight from China.
Now if the arduino was 10x cheaper, the entire value proposition and application space changes.