Why do lightmaps default to 1024 max size by default?

Title. Is it due to a limitation? Mobile can package up to 2048

you can set it in world settings under (not in but right under) lightmass settings there’s an arrow you need to click and it will expand a few more settings

I asked why is it 1024, not how to change it.

I don’t think there’s any specific reason, it’s just a reasonable texture size to pack multiple lightmaps into. A larger atlas can fit more lightmaps into it, but can also be slower to stream compared to multiple smaller ones. You also have to consider the other unique atlases generated by default, like LQ and HQ lightmaps and skylight occlusion.