This was never said and shouldn't be assumed. There's a lot you can say about adding X feature by just from having a fairly abstracted understanding of how the game works.
Latency (lag) needs to be considered because in any dragon mount situation you are combining your latency with the latency of another person in a way where lag will be more apparent to both involved. No matter how you are "treated" by the server the rider won't get position updates until the server receives the new position from the other person, processes it, and then relays that information to you. If either connection starts to hiccup you're going to see artifacts that detract from the experience. As an example, if you ever play Horizons with another person in the same room you'll notice a sizable delay if you watch yourself move from the other person's computer depending on your internet connection. It doesn't take insider knowledge to observe that.
It's certainly possible to overcome such obstacles with time and money, but this idea combines a lot of effort from many sources. Properly adding dragon riding would require significant new artwork in new animations, modification to the client to provide a basic interface for the feature as well as handling how to attach a biped to a dragon model, no small task, simulation work regarding how characters should behave when riding, and possibly changes to the server to better handle position updates.
Finally, there are considerations to be had as to how giving bipeds access to dragon riders would affect gameplay. Bipeds without access to a dragon are going to perceive a sudden disadvantage. One thing I've observed for a while is that in all MMORPGs if you give any group even the most insignificant advantage you're going to upset someone who does not belong to that group no matter the circumstances. People will bicker amongst each other when a dragon refuses to give a biped a ride or acts maliciously.
Think of how easy it'd be to grief people with this ability. Dragon goes to New Trismus and gives out free rides to new players. New players bite because who wouldn't want to take a ride on a dragon? Dragon drops player into the ocean. Player is upset, dead, and isn't having fun. There's no way to prevent this. No matter the circumstances you're giving a player the ability to kill or seriously screw with another player at their discretion. That type of behavior is pretty darn funny from a dragon perspective, but it will create all kinds of hassle for customer support and hurt player relations in general.