If Rousey wants the Cyborg fight why doesn't she just go up to 145? Cause she started her career there so what is the issue? 
just saying...
1-Neither Rousey nor Cyborg are street fighters that they would meet at a parking lot and fight
2-Rousey is a contracted fighter of UFC and Cyborg is not. UFC decides who she fights and she has never dodged any fighter she was pitted against.
3-UFC has only 2 weight categories... 115lb and 135lb. Dana White declared a number of times that if Cyborg wants to fight at UFC, she has to make the weight.
BTW, Cyborg's real weight is 175lb, on Thursday at the weigh-ins for Invicta FC she barely made the 145lb but on Friday the fight night she was already 170lb. The fighter she beat in 45 seconds was also beaten by Rousey in 40 seconds early in her career.