The short answer isWhy not give the player the option to change costumes ingame?
The long answer is the way to get around it requires a lot of code and a million more images. While changing ingame would be nice, spending months or possibly years just to add clothing options in existing images is really poor use of development time.Cause Ren'Py use prerendered images.
Sure we can have clothing options in specific cases like office punishment, but technically it's not one scene with 3 clothing options, it's 3 scenes made from scratch to get the images and hence 3 times the disk space. If clothing options are done in general and possibly with combos of multiple options, like 3 people each with independent options, the number of images for a single screen will become huge in no time. This means options should only be done if they really add value rather than whenever it would be nice to have.
Sure it would be nice to do real time rendering and that way have clothing options and animation instead of still images. However since Ren'py doesn't support that, getting support for that means starting over from scratch with not only a new engine, but also new graphics as the current graphics have no 3D awareness and as such can't be made to swap clothes even if the engine adds support for doing so.