Kes wasn'st spending all her time in sickbay or in the Hydroponic Bay. She spent a lot of time on the bridge hand had her fair share of being on away missions and visiting other planets.
When it comes to exploring the ship and the holodeck, don't forget that it was one and a half month between "Caretaker (Stardate 48315.6=26 April 2371) and "Parallax (Stardate 48439.7=10 June 2371). She had a lot of time to explore the ship and the holodeck during those weeks.
Not to mention that there were no similar development of Neelix and the Maquis characters either. They joined the Voyager crew and BOOM! Already in "Parallax" they were all settled in in their roles at the ship.
Comparing Kes with Seven is unfair because the show shifted focus when Seven came in and became the main character of the show. Kes was never expected to be the main character in the same way Seven was.
Kes was a great character who personified the spirit of Star Trek with her will to learn and explore. That spirit disappeared when she was kicked out.
Not to mention that the show was better in the first three seasons when all the characters had their fair share of the spotlight. In the later seasons, it was all about Seven, The Doctor and Janeway.