Why do I need to assign static ip address to the bridge and not to the physical interface? [duplicate]
That's how bridges work.
When you bridge devices together, the networking configuration applies to the bridge since it is representing all of the connected devices.
Therefore you must apply the network settings to the bridge, and not to any of its constituent devices.
That's just the way it works.
eth0 is part of the bridge, it is no longer it's own interface and the bridge interface 'owns' it. So to have an address, you need to configure it on the bridge, not on eth0.