i just wondered... why don't they attach the shuttles to the ISS to increase its size instead of putting them into some museums?

Would be actually a good idea. Especially since it could serve
as a permenent emergency escape craft.

Maybe the Americans who park it there, dont want to be taken back
by a Russian Craft... ist against the land of the free, and stuff.
(or not to be a storage place for toiletpaper and Vodka)

Would even have enough cargo space to save some experiments or data, in case they have to evacuate... Or something.

And then in the far future they could upgrade them with new engines, installation would be far easier in microgravity than on earth.

Nevermind, in the end it comes down to money, and it's just cheaper to keep them on earth than to send them up.

If you'd permanently attach a shuttle to the ISS, it would block the passage other spacecrafts would use to connect to the ISS. So that would require extending the actual ISS with more passages or build more passages into the shuttle (somehow). I think that wouldn't be worth the effort.

Also I don't know if the space shuttle is suited for staying in space permanently (because of oxygen and stuff).

yes, probably it would have required some extending with more docking ports.

but the european columbus module did cost 1.4 billion euros for example. i can't believe that it wouldn't have been cheaper to turn a shuttle into a useful module if they aren't needed anymore for anything else anyway.