Who said that it would be?
It's just not tested - and therefore you're more likely to be the first to encounter bugs, but it should work. Case in point, for a (very short) while a PPC64LE buildbot was used to build OpenWrt, which worked - but 'failed' when producing the imagebuilder binaries (well, it successfully built them, but obviously for PPC64LE and not x86_64, which made them rather useless for the public at large). Proving if it actually does, or what the issues are is up to those who actually want to use that hardware.
Why would it be weird?
At least so far, there was not a single ARM device available to mere mortals on a budget that could compete in terms of build (CPU, I/O, RAM) performance with even an aged mid-level x86_64 system. The mythological ARM servers which have been promised to conquer the market $next_year for at least half a dozen years still don't exist (unless you're a huge company with deep pockets to drive development yourself). While the RPi4 or Rockchip RK3399 are certainly a step into the right direction, they still can't really compete with an entry level ryzen system, even more so once you look at RAM sizes and I/O options (SATA, PCIe, …).
In terms of availability and performance, the Apple M1 could be a game changer, but it has just entered the market half a year ago - and prices and lacking Linux support don't really make this a straight forward choice at this point. I'm sure someone will try building OpenWrt on their shiny new M1 one day, just don't expect anyone to spend their own money to do it for you (while it's still new and expensive).
The same goes for renting CPU cycles on ARM based data centres, it costs money (the project doesn't really have an endless supply of) for rather little reward, it's not cheaper than renting x86_64 based resources, nor any faster, nor providing any other tangible benefit at this moment. The official binaries would still have to be built on x86_64 in order to provide imagebuilder binaries that are actually useful to the enduser - so trying it elsewhere would be a rather pointless exercise of proving that it works (which it should in general, you're just more likely to encounter issues <-- patches welcome).
If ARM is here to stay on the (consumer/ prosumer/ developer desktop and workstation) market, it needs to become competitive. What does this mean? Well, you'd need to be able to buy such a device that's roughly on par (performance, RAM, I/O, storage) with contemporary x86_64 systems, for comparable prices (+/- 15%-20% at most) - and it must also scale with the x86_64 offerings (from entry level budget to mid- to highend consumer devices; let's say i3, i7, ryzen competitors). And something like SBSA compliant devices must finally become available to mere mortals on a private budget, meaning that you can install any of your preferred linux distributions without having to think twice.
Apple with the M1 is certainly one alternative here, but at this point not really the natural choice for anyone wanting to do develop linux systems - you'd have to pay the early adopter's tax twice, once at the sales counter, another time trying to hunt down all the little warts and kinks before you get the build environment working.
RPi4 and RK3399 (etc.) have certainly paved the way to consider looking into ARM, but their performance is far from sufficient to do anything but one-off proof of concept builds. Sure, you can do that, if that's what you want to prove - or you could do actual development on OpenWrt.
Time will tell if ARM can become competitor (or of nVidia smothers the competition), once it does, it will find the way onto developers' desks quite naturally.
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Disclaimer: I'm not an OpenWrt developer and can't speak for the project. But to the best of my knowledge, none of the OpenWrt developers are currently in possession of Apple M1 hardware.
