Not only that but look how it forms a freaking fishing hook on the end like a barb. Yikes!!!

Assumably also for manufacturing and safety reasons. You don't want the tip of a needle to shatter inside you, softer steel won't do that. And it's a little bit easier to manufacture with softer steel as well.

Needles were autoclaved and re-used once upon a time, so it should be possible. But disposable needles are probably made of softer material than reusable ones.