Because the downsides of strictness are great as well. Laziness lets you "decomplect" production from consumption, with a lot of safety, most of the time. I think few who've spent most of their time working in strict-by-default languages recognizes the pain of it. Those who do write opt-in lazy language properties like generators.

So the question becomes whether strict-by-default or lazy-by-default is better given that both kinds of evaluation have their place. I personally have come to believe that lazy-by-default is nicer since there are fewer places where I really demand strictness... but it also basically forces your language to be pure.