I know that this sounds way off, but why not just replace hips before they get broken? So many old people break them anyway.

That's an interesting idea but not all that practical. First of all, not all seniors do break their hip, so it would become difficult to have to decide who would get a replacement and who wouldn't. But, setting that all aside, this type of question shows that there is a misconception about the risks of surgery.

Any type of surgery has its risks. Not everyone is healthy enough to have a major surgery like a hip replacement. Illnesses such as heart disease, asthma, and diabetes, can cause problems post-surgery. Hip replacement surgery requires that a patient go under a general anesthetic (risky on its own) and be subject to many of the potential surgical complications, such as infections, blood loss, and malfunction of the implant, to name a few. Then after the surgery, the patient has to be confined to bed and chair for a while and then undergo physiotherapy to regain the strength in the leg and hip. For some people, this is easy, for others, this is much more difficult.

So, while the idea is certainly an interesting one, it's not a practical one.