The VESA thing is indeed a drag and the VESA mount option is either more popular or very popular because there are periodic sales on Studio Displays, but many of these sales specifically exclude the VESA mount version either because those are already selling or they don't have any of them available to put on sale.
There's no kit because the stand is built into the frame of the Studio Display, but from what I've read, you can take your Studio Display to an Apple Store and they'll convert them to a VESA mount version for a fee. When I was shopping for an Studio Display earlier this year I considered this option as the VESA mounts were out of stock and the regular versions were on sale. If I felt like the hassle of bringing a monitor in for service, I could have saved a little money in the process.
Instead, I paid full price. I waited and jumped on the VESA units as soon as they returned to stock.