iOS, with its single hardware manufacturer, has been 64-bit only since 2017. Android has a million moving parts split across a bunch of different companies, so getting to 64-bit only is going to be a long road. Getting there is worth the effort, though, with a promise of increased performance and additional security features.
It’s not just that. AArch64 is a different architecture than AArch32. For one, there are more registers available on AArch64. Also, 64-bit architectures also let you do tricks that are not available or are indefensible with the smaller address space that a 32-bit processor provides.