For one thing, multiple teeth can adjusted individually for wear and alignment.

You said in the comments:

i know there are differences. My question was maybe a bit unclear. I didn't want to ask why there are teeth instead of bones, but why there are multiple teeth instead of one large spanning the entire jaw. Besides, parts of teeth (dentin, cementum) are not that different from bones. I never questioned the presence of the harder parts (enamel).

But this conflicts with points in your question which were intended as the argument against teeth of any kind, singular or multiple:

• teeth are quite fragile when impacted and break easily
• gaps between teeth are difficult to clean and risk decay

Can you imagine what would happen if your ONE tooth broke or decayed?

i know there are differences. My question was maybe a bit unclear. I didn't want to ask why there are teeth instead of bones, but why there are multiple teeth instead of one large spanning the entire jaw. Besides, parts of teeth (dentin, cementum) are not that different from bones. I never questioned the presence of the harder parts (enamel).

For one thing, multiple teeth can adjusted individually for wear and alignment.

However that clarification in the comments conflicts with with points in your question which could be construed intended as the argument against teeth of any kind, singular or multiple, especially singular.

• teeth are quite fragile when impacted and break easily
• gaps between teeth are difficult to clean and risk decay

Can you imagine what would happen if your ONE tooth broke or decayed?