why you cannot boot to active partion of windows after installing linux

this is for legacy(bios; but still popular) environment. for UEFI system, this does not apply.

the reason is, grub is installed in MBR directly. and grub does not care about active partition.

if you want to use bootloader of windows, try easyBCD. it tricks grub with virtual mbr (.mbr), so you can load grub from windows bootloader.