Why can't you just tile over old tile?

You can do that if you want. Most people don't because it makes the wall thicker and heavier. Usually you need to replace about a bazillion other things like the tub and plumbing or wiring. So it's kind of a matter of not making too much work for the next guy.

It is a matter of working clean and being sure that all is square, the wall structure secure and supported, no mold, leaks, weird or live electrical wires hiding.

I've open shower walls to all of the above and found rags wrapped around live wires, rat, mouse and bird nests.

Sometimes a place is quite up to par and you can just tile over what is there. I haven't done that, as I look for maximum return on redos and like a streamline finish and anywhere there is water I want to avoid later problems of mold and dry rot.

i have done the same thing- just have to watch doors to make sure the new tile u r installing is not to tall & u can't shut the door

Since the tile is a smooth surface, problems will occur with new tile sticking and staying on. Remember, you need a good adhesive so the tile will not fall off if moisture gets behind it or the weight is too much. That would be a mess. It is best to spend a little more time removing the old and adhering the new.

Good that all is well. Now you'll have to decide if you want to tile over tile. It will stick and work for you, just seal the out of the bottom layer and have good expansion joints at corners and bottom.

Sometimes it it good to cover ugly. And have it be a manageable project! (If it was my project I would use tiles in the thin side that butt closely.) I think that will help your results with weight/thickness and possible cracking:shifting of the two differing surfaces.

Have fun!

Weight and potential collapse of wall board.

Ugh! Something to consider