Why does FPGA PCI Express Tx output have on-chip termination of 100 Ohm?
There is termination on the transmitter end due to the same reason, to terminate reflections that arrive back to the transmitter for some reason, and also to drive the 100 ohm transmission line at 100 ohm source impedance, as otherwise the current mode output would have extremely high impedance.
As PCIe lanes are AC coupled, it is also there to bias the current-mode driver output correctly as the output drive current is such that it requires double termination to output correct voltage swings.