A. Housing is one of the basic requirements, and there are a variety of reasons why you wouldn't be able to time it like that. E.g. ended a old job, started a new job, marriage, birth, death.

B. Even outside large life events, many people don't want to disrupt large holidays with a stressful move. Ditto for school terms.

C. Houses look and show better in summer that winter. Seeing a healthy yard instead of bare trees, snow, and brown grass is good for value.

D. Often, the person buying a house is selling a house at the same time. Your new house costs 10% less in the winter, but your old house sells for 10% less as well. (Still a good deal if you are upgrading; just less of a good deal than you might initially think.)

E. If buying a house is absolutely conditional on selling one, winter is a somewhat riskier time since the market's volume is lower.