If you look at GAS manual https://ftp.gnu.org/old-gnu/Manuals/gas-2.9.1/html_chapter/a... almost every other architecture has architecture specific syntax notes, in many cases for something as trivial comments. If they couldn't even decide on single symbols for comments, there is no hope for everything else.
ARM isn't the only architecture where GAS uses similar syntax as developers of corresponding CPU arch. They are not doing the same for X86 due to historical choices inherited from Unix software ecosystem and thus AT&T. If you play around on Godbolt with compilers for different architectures it seems like x86 and use AT&T syntax is the exception, there are a few other which use similar syntax but it's a minority.
Why not use same syntax for all architectures? I don't really know all the historical reasoning but I have a few guesses and each arch probably has it's own historic baggage. Being consistent with manufacturer docs and rest of ecosystem has the obvious benefits for the ones who need to read it. Assembly is architecture specific by definition so being consistent across different architectures has little value. GAS is consistent with GCC output. Did GCC added support for some architectures early with the with help of manufacturers assembler and only later in GAS? A lot of custom syntax quirks which don't easily fit into Intel/AT&T model and are related to various addressing modes used by different architectures. For example ARM has register postincrement/preincrement and the 0 cost shifts, arm doesn't have the subregister acess like x86 (RAX/EAX/AX/AH/AL) and non word access is more or less limited to load/store instructions unlike x86 where it can show up in more places. You would need to invent quite a few extensions for AT&T syntax for it to be used by all the non x86 architectures, or you could just use the syntax made by developer of architecture.