It's a really, really slow computer. The Pi 4 is something like 10x faster than the original Raspberry Pi, but that was also a really, really slow computer. I remember not wanting to set up a cross compiler once, and recompiled ntpd on one. It took more than 24 hours. The same build runs on my desktop in seconds.

It is good for things like reading a sensor and sending it to some server, but so are ESP32s which are cheaper. You are paying a heavy price to get video output and Linux compatibility. In the current era, it's a bad microcontroller (expensive!) and a bad desktop computer (slow!). When it came out, most people were using $25 MCU boards, so $10 extra to run Linux was a great deal. These days, a massively more capable MCU dev board can be had for $4 (if you want free 2 day shipping, less if you can wait).

It's relatively cheap, and it seems to be supported fairly well (loosely, "good"), so that's two points of the triad. The Law says we can't also have "fast", but that's a reasonable trade-off for those who prioritize "cheap" and "good".

I mean, if you want a build server that runs at Core Duo speeds, you can probably find a used Core Duo workstation or laptop on eBay for less than what a Raspberry Pi costs. While we often talk about how Moore's Law is dead, go back several generations of computers and you'll be impressed at how slow they are. (I'm regularly impressed with how slow current generation computers are!)

I use my pi 4 as part of a build farm with a 2cm tall passive heatsink. It never goes above 65c.

64-bit kernel, or 32-bit?

32

Well then I'd say sure, of course you need cooling. But I'd also guess that you're within <1% of Rpi users who actually need cooling, and I'd guess that's also why Rpi doesn't come with a radiator by default - vast majority of users simply don't need one and it would increase the cost.