Definitions vary. The formal Regency period was nine years. But, from Wikipedia: "The term Regency (or Regency era) can refer to various stretches of time; some are longer than the decade of the formal Regency, which lasted from 1811 to 1820. The period from 1795 to 1837, which includes the latter part of George III's reign and the reigns of his sons George IV and William IV, is sometimes regarded as the Regency era, characterised by distinctive trends in British architecture, literature, fashions, politics, and culture."

because the constituencies involved (of both the characters in the book, and the modern audience that consumes them) prefer the absence of a strong federating remote figure, to the ability of being able to imprint/insert themselves as participatory observers of the events.

lack of a strong central nexus gives fantasy writers and readers the opportunity to engage with the story and period on their own terms, instead of being subjugated to what some accidental monarch wants the era to have been.

There really wasn't much of a difference for them in terms of actual actions aside from more gentry officers actually being in the field. They still could and did have those parties before and after. It would be a matter of being upshown instead of controlled.