Sex toys have come a long way, fellas! Don't worry about what women are using unless those women have invited you to witness! (My current fave thing is shaped like a mouth with a tongue sticking out and I'm not even a lesbian!)
That's not what the inside of a vagina looks like. The answer is that evolution caused that shape to be one that's good at causing a pleasure sensation from penetration of a vagina. However, other shapes also exist and also do this. The penis shape is just the classic option.