It is only because of the setup of the game. I hope in the future the Devs take on a more long-term setup that allows for almost no need for power creep, like Magic the Gathering card game where they have their “standard format” that only allows cards from the most recent year to be played. For PvP the Devs could do something similar where they only allow monsters released in the last 12-18 months to be used then a select group of other monsters. We’d also get a free legendary every few months that can forever be used in PvE but will only be allowed in PvP for the next 12-18 months and never again. These free legendaries can be designed to encourage certain metas.
I wrote all about that in KD's Recommendations for the game, so you can go read it for a better explanation.