Electronic – Why are Ethernet jack center taps connected to VCC

100% sure answer may need much digging but

It is almost certainly for safety reasons.

A current transformer output produces its result in splendoured isolation - it floats relative to the universe (or ground or line or ...). Stray leakage and capacitive coupling MAY cause it to float to undesirable voltages so referencing it to something is a good idea [tm].

Connection to phase would achieve this - it will still function the same BUT this may make your eyes water or worse if the insulation is not perfect or you find some exposed part of the circuit to touch.

SO connecting it to ground at some point in its circuit may make your days longer on the face of the land - or those of your client or family or ...

IMO it's a pull-up resistor, TXP and TXN is just a open collector output that ties the line to GND level. The total current is now the current sum from both pull-ups. One goes directly from pull-up to ground, the second goes from pull-up trough a transformer and to ground. The receive channel is the same, since it is auto-MDIX. Therefore you can use a cross over cable and the TX and RX channels got swapped automaticly.

EDIT:
I said IMO, there is no detailed description about the IC output. Here is the picture of CML output type. Do the 50 ohm pull-ups make sense now?

EDIT2:
The ethernet transceiver IC differs from manufacturer to manufacturer. It is necessary to use the template or demo board as recommended by the manufacturer. If some manufacturer uses centre tap tied on +Vcc and some else is floating, .... its due to the IC characteristics and can't be swapped or used an universal schematics that will fit all ICs and all magnetics.