This is due to the half-duplex and shared medium nature of Wi-Fi. The device can not receive and transmit at the same time, and only one device may transmit on the medium at the time as all mesh nodes are in the same channel.
Therefore if say you have a network with nodes A, B, C connected as below
A ←–> B ←–> C
And A is talking to C by hopping through B, then B must receive from A, then transmit the packet to C. It is not possible for A to keep transmitting while B is transmitting the packet to C. Therefore you’ll roughly halve the available bandwidth when you add a hop.