6 months is a way to lock people into a role. People with a 6 month notice periods, generally need to resign if they are unhappy with their pay or the role and then only look for new roles nearer the 3 month mark. The job market can change pretty drastically in 6 months, so be sure to factor that into your thinking. If possible use your probationary period to exit the role early if you think it won't work out.

I've never heard of 6 months for senior development roles in Aus/Uk/Ireland, it is mostly lead, staff or principal roles that have longer notice peroids. It can also be tagged onto a relocation package if they are sponsoring you and/or paying for your relocation.