Because I believe the benefits outweigh the risks.

The benefits to my children include not being killed or crippled by polio and having a mild case of a childhood illness rather than a severe case.
Also we have traveled to less developed countries than Australia and hope to do so again. I wouldn't want my children to live with the guilt of infecting children who don't have the nutritional benefits to throw off something like measles. Some diseases that are rare in Australia are common in other countries and if we are traveling in areas with a higher incidence of disease I prefer my children to have some measure of protection.
I prefer to provide herd immunity rather than to rely on other people to provide it for me. If everyone relies on other people to provide immunity pretty soon herd immunity will be a thing of the past.
I have never yet met a scientist with a genuine understanding of micro-biology and disease control that has chosen not to vaccinate although I know a few who have followed different schedules that better suited their circumstances.

I do wish that vaccines weren't given in bulk and that each disease had it's own vaccination rather than being combined but I do understand that from an economic perspective that it makes more sense to have the kind of vaccination schedule that we do have.