Root != ability to flash ROM
Root = getting Admin access on your Windows machine.... You can modify files in C: \ drive(and the potential to delete some essential files and your computer cannot boot correctly), perform MORE actions than a regular user
Most phones have a locked/encrypted bootloader. This is akin to saying: device will only (boot)load this specific version of android configured by the manufacturer(or computer can only boot/load Windows 8 configured by the manufacturer)
With a locked/encrypted bootloader, you cannot install any Operating System. You can only use what the manufacture gives you (eg Windows machine, can only update from WIndows 8 to WIndows 10 configured by the manufacturer, cannot install Ubuntu or other Mac OS X)
some phones have unlockable bootloaders(eg nexus series). Some phones have exploits to bypass the locked bootloader (eg
lg loki exploit)