I don´t quite know the reasons, but even a 2 bit image is treated as 24bit internally.
I´m not sure about a png 32 bit (with alpha).
The former I read in another post, maybe someone can explain better.

It may have something to do with open gl.
I think the only benefit is hdd space too.
I could be nice to have indexed colors (256 colors or less) and the ability to shift pallete similar to Amos on amiga, ye the oldschool stuff.
But I´m not sure this would be necessary as a feature to all users and the actual trouble to have it going that way.