I would suspect it is a case that folks just aren't looking hard enough yet. There is also the massive fragmentation in the ARM space. Something like M1/M2 being a much more consistent target.
It could also just be that they have designed them better but time will tell. A lot of things looked really secure until they suddenly were not.
ARM chips are mainly much simpler. The fight with Spectre-like side-channels is a fight against thermodynamics we cannot win, only mitigate.
Any performance improvement relies on the patterns of data also exposes the data itself. Intel and AMD wanted to make faster returns by analyzing the patterns in the return instructions by putting a branch prediction logic. When they do that it behaves differently depending on the data hence it is "observable" to outside world.
If any ARM manufacturer implements a similar feature, it will be vulnerable. There is only one question: When will it be practically exploitable?
Links to ARM FAQs and other docs below. Note that ARM's list of vulnerable implementations doesn't account for third party designs eg Apple CPUs.
https://developer.arm.com/documentation/102587/0102/General-...
https://developer.arm.com/Arm%20Security%20Center/Speculativ...