There's a thread on another site where someone had a similar question. Apparently the M2 MacBook Air does not offer this choice (under either Monterey or Ventura), where some Macs do. Someone mentioned a workaround … but I have no idea whether it is safe or effective.
https://apple.stackexchange.com/questions/460238/why-no-3200x1800-resolution-on-an-m2-macbook-air-on-a-5k-display
Here's one where someone had a similar question about the M1 Mac Mini.
https://www.reddit.com/r/MacOS/comments/11hiimw/whats_the_max_scaled_resolution_on_the_studio/
If you think Apple should offer a 3200 x 1800 choice when using the 27" 5K Studio Display with the M2 MacBook Air, you could send feedback to Apple. I would suggest including the links in your mail. The thread at the first site says that "a senior advisor at Apple was stumped" by the issue, so you probably want to provide all of the context that you can.
Product Feedback - Apple