I had a little chat with the author of age about this. Rclone crypt is in fact very similar to age - the encrypted data in both is chunked into 64k chunks in a very similar way so they can be streamed.
The differences come in the header which is variable length for age and fixed length for rclone.
A fixed length header for rclone lets rclone calculate the length of the file without having to read anything from it which is super important for filing systems which need to know the size of things.
It would be possible to build an age backend for rclone but it would have the disadvantage that if you did rclone lsl so show sizes of the files, then rclone would have to open each file to read the header from it in order to calculate the size of each file.