The interest rate in many countries are very low now.
Many people buy bigger house because they can afford the monthly mortgage.
They do not like to think about the increasing interest rate. While they know that they are getting a dirt cheap rate, they hope that the interest rate will always be low.
If the interest rate doubles, can you still afford the house? What if it triples?
It is not too far fetched, because interest rate was very high many years ago, and it will be high again in the future.