Comment by akoboldfrying

I think you're missing the point. The 8087's double-precision limit of ~15 decimal digits wasn't arbitrary -- the purpose of that device was to perform practically useful computations, so that amount of precision was chosen by the designers as a reasonable engineering trade-off.

IOW, the likely reason for not storing more mantissa bits than that is someone involved in designing the 8087 determined that even NASA doesn't need more precision than that.